'71 Challenger blew fusible link after servicing the grounds.

[B and A Bill]

So, I bought the car on a Sat. Probably put 200 miles on it between Sat. and Sun. morning. It had a few electrical issues, but, I didn't think anything was majorly wrong.
A little preface:
The headlights were very dim.
Pass. taillights didn't work (bulbs).
No dash/interior lights (orange wire seems dead).
Fuel level is off, and sometimes doesn't work (cycling the key will usually wake it up).
I am sure there is more, but, I wasn't really compiling a list, LOL.
At some point the battery made its way in to the trunk. The PO said he did it because he wanted to mount a remote dual oil-filter where the battery tray was (never happened).
Okay, so the battery is in the trunk, whatever (for now). The main ground from the battery to the body was POOR at best. Just a ring terminal to the open tab/hole at the rear of the wheelhouse pinch weld. The engine to body ground was no better. A positive (red) cable from the rear of the head (not bad) across the intake with the alternator harness, to one of the horn bracket mounting bolts (terrible!). The paint was never even cleaned off. Those were the only two grounds. I am surprised really that it would even start!
I don't know what is supposed to be there.
But I assume: negative battery cable to engine. Ground strap from engine to firewall. And small ground from negative battery terminal to inner-fender. At least?
Anyways, I checked the condition of the charging situation Sun. night, and at idle there was 12.7v at the battery. Not good. I immediately suspected the grounds.
Ohh, I guess I should mention, hot side of the battery goes directly to hot starter post with 1/0 under the carpet. No fuse, no breaker, no disconnect! 

So, I removed the hokey engine to horn-bracket ground, and built a new 1ga ground from the rear of the pass head to the pass inner fender. Properly terminated.
Then moved the battery ground to a factory hole in the hinge-support. Cleaned the back-side down to bare metal, and terminated properly.
That was it. That's all I did! But, upon reconnecting the battery (key off, it was in my pocket) the fusible-link to the main feed instantly went up in flames.
And, left me standing there scratching my head, because I didn't touch anything that could have caused this. It had to be a pre-existing condition, that wasn't an issue because the grounds were so poor? (seems highly un-likely). Or one of the biggest timing coincidences I've ever experienced! Pics of the aftermath:

[B and A Bill]

You can see that the terminal in the outside harness plug was quite warm (discolored), and the insulation on the wire on the inside of the bulk-head was pretty much boiling.
The wiring in this car is surprisingly un-molested! That is a good thing! There is a fair bit of green-death in the terminals in the bulk-head (that is a bad thing).
But, nothing like I am used to seeing in '70's Pennsylvania Dodge trucks! HAHA

I have only been able to do a limited amount of sleuthing since. Simply doing continuity checks with a DMM. At first it seemed everything was shorted to ground.
Specifically the main feed under hood, both sides of the fuse-block, and battery/accessory/ign.1/ign.2 in the dash side of the ignition plug (that was all I checked right away).
Later on a few hours, it seems only the accessory side of the fuse-block was shorted, and the battery side was now okay? IDK. it was dark, late, and I was tired.
I'm not even sure a continuity check with a DMM is going to tell me much? IDK? Electrical trouble-shooting is my weakest suit!

[usraptor]

I'm not expert either when it comes to electrical, but it seems to me that you have a direct short to your positive battery cable.  First thing I would do is isolate the positive cable and make sure it's not grounded somewhere.  Along those lines, seems like now would be a good time to move the battery back into the engine compartment where it belongs.   

[JS29]

I see a couple of bad connections there. The key in pocket is the thing that troughs me.     

[RJChallenger]

The problem with direct shorts like yours is "any and or all" evidence of a short can be destroyed in the initial meltdown . Plastic insulation may have melted , now insulating / braking the short to ground. Or a "ground wire" that was touching the positive wire may have completely melted, no more short to ground. You can test with a meter all you want and never see a problem until you remove all the tape or  whatever is covering the wires exposing any melted wires. Then you could test each and every wire for continuity. Good luck

[Chryco Psycho]

When I do a battery in trunk I run a 1/0 welding cable to the starter with no fuse as both ends have power  & a similar gauge wire to the transmission for ground , that way the engine is the main ground not the body as most of the load is the starter . I use a 8ga ground from the engine to the body at the rad wall to pick up the body grounds .
All attachment points need to be clean as you know . I would clean all of the bulkhead terminals & use di-eletric grease in there to protect those connections .
Another must is headlight relays which remove more load from the bulkhead connector the headlights have a ground near the stock battery location on the Rad wall , that has to be connected   & you should connect both wires on the ammeter to 1 terminal .
Factory had a double negative cable , 1 pigtail to the rad wall where the headlights ground & the heavy cable to the left cylinder head .
Your orange dash lights may be a bad dimmer switch beside the headlight switch you can jump it to be sure .
Now you need to find the problem

[Chryco Psycho]

so the fusable link goes through the firewall to the ammeter & then to the ign switch in the column SO I would trace that circuit looking for the short , you could unplug the ign switch plug at the bottom of the column & see if the Ohm meter reads a short still from there it goes to the fuse block , 1 side is battery /constant power the other is switched as the key was in your pocket it can only be on the battery side .
I hope this helps , I can call if you need me to .

[B and A Bill]

Thanks for the reply Psycho.  I know all about the ammeter. Plan to swap one wire as soon as I figure out how to get my hand up there! LOL
I am a bigger dude, and did not realize until now just how cramped the dash in the E-bodies is. I am used to B/C-bodies and trucks. Much easier to work up in there.
The plan is to move the battery back where it belongs. Weight transfer is pretty darn low in the list of worries! I ordered a new tray today.
And, the wiring gets even better! The attached pic shows the epic condition of the +12v wire feeding the hot terminal of the starter relay from the hot post of the starter.
It's a classic!  Don't worry, I'll fix it. I may not be great at trouble-shooting. But, assembling I am pretty darn good at.

[Chryco Psycho]

I meant to mention that wire also 10 ga from the starter to the start relay to feed the car but the fuse is after that so you still have a problem past that wire

[B and A Bill]

New battery tray should be here tomorrow. I aborted figuring these issues out as is.
Pulled the ground from the hinge-support. Pulled the 1ga that was running from the rear-mounted batt. to the starter terminal.
Scored a nice chunk of cable there, LOL. I will use some of it to build a new starter cable.
But, I am going to get the battery back where it belongs before I go any farther with this.

[B and A Bill]

Okay, the battery relocation back to where it belongs is complete.
The battery grounds to an un-used 7/16-14 boss on the front of the block just under the drivers side head with 1/0 cable.
There is an 8ga from the battery negative terminal to the headlight harness ground.
And, a 4ga from the rear of the passenger head to the passenger inner-fender.
Grounds are good to go! (I warned you guys I am not a purist, LOL)
Battery positive goes to starter hot lug with 1ga. and starter solenoid hot post with 10ga.
Then I made a little jumper/test fused wire to replace the fusible link temporarily to troubleshoot why my fusible link went nuclear in the first place.
I started with a 10amp fuse. I noticed some sparky-sparky while connecting it (to the half melted original fusible link terminal on the headlight harness).
But, the fuse lived, and the car was live. Started right up. Seemed all systems go. Fuse popped about 30 seconds in. Kept running off the alternator though.
It was at this point I noticed the brake lights were always on. Sure enough! Looked up under dash, brake light switch is broken! That explains the sparky-sparky.
Un-plugged brake light switch, replaced fuse. No more sparky-sparky. Started right up again. Fuse lasted until I turned headlights on. Then POP.
Okay, now we are getting somewhere. Replaced 10amp fuse with 20amp. That fuse is hanging tough and still there.
Managed to get brake light switch rigged well enough for this session (Not going to drive it until replaced). But, it seems okay now.
Battery at rest is 12.8v. Engine running, I see 13.2v across the battery. But 14.2-14.7v from the hot post on the alternator to ground. So, typical Mopar harness BS.
Still don't know why the fusible link had the melt-down with the key in my pocket after the first ground maintenance session? But, at least I made progress tonight.
More tomorrow (I hope).

[Chryco Psycho]

Well , good progress . not sure why the link melted down but at least the 20 amp fuse is holding now 

[B and A Bill]

The plot thickens. Now, any time the light switch is on, the light buzzer is on. This screams ignition switch to me. But, I don't quite understand how the buzzer circuit works.
I also don't know, are there two separate buzzers? One for the key-in, and one for the lights? (key in does not buzz, ever)
I also popped the 20a fuse by turning the headlights on. I am sure a big part of this is the bulk-head. But, there is more going on.
I'm using the wiring diagrams from the sticky here. The under-dash diagram is for a rallye dash (which I do not have). And, I don't know what actual differences there are?
And, the under-hood diagram does not seem to coincide with the under-dash diagram at the bulk-head? making it a little cornfuzing.

[Chryco Psycho]

I would install the headlight relays right away this will take a major load off of the bulkhead & the switch as well .
The lights warning buzzs with the door open ,
Under dash is pretty similar , you have most of the same wiring & features , with the exception of the tach & clock , , you still have a voltage reg for the gauges limiting it to 5 v .
Under hood the wire colors should be the same but the positions can change year to year

[B and A Bill]

Okay, back on the electrical in this pile. I have only had front parking-lights the whole time.
No tail-lights. I did have brake & turn in the rear, and turn in the front. But, all 4 side markers, and the tail-lights were dead.
So, I replaced all the side marker sockets and bulbs (and lenses). That fixed that. Tail-lights seemed to be a ground issue that fixed itself while I was rooting around.
I also replaced the brake light switch because it kept falling apart. At this point, all the exterior lights worked. Horray!
Until...........now it keeps blowing a fuse, and I loose all the parking-lights. Dang it! The fuse that blows is labeled "MISC". It blows after about 5 minutes of turning the lights on.
I have found all the schematics I could find on-line. And, best I can tell, this fuse is for tail-lights and the lighter? Which immediately tells me there is something wrong with the lighter circuit.
So, I disconnected it. Problem still persists. Quick and simple test with fuse removed doesn't really lead me to believe there is a short any where in the light harness.
And, I am sure the tail-light housing needs some attention to make sure it is grounding well. But, I don't see a bad ground blowing fuses?
Not sure where to go from here.