Brake light switch questions

[blown motor]

I'm troubleshooting why the brake lights on the 78 Dodge truck don't work. Two questions: 1) what's the green wire? Is that a ground the needs to be connected somewhere? 2) which side of the switch is live? I want to test for power at the switch. I tried both sides, nothing.

[Bullitt-]

  I don't know anything specific to the '78 truck, can only find a good diagram for a '79 at https://mymopar.com/wiring/.
  Checked RockAuto & they list the same turn signal switch for both '78 & '79 so assuming they are the same I believe the GREEN wire has been added and not necessary for brake light operation.
In the '79 diagram if you follow the wire F48-18GR (i think GR stands for GREY) from the "Stop Lamp Switch" on page 8-152 to page 8-153 it leads to the fuse box which makes me think there should be 12Volts on that wire. The other wire S17-18WH (I think WH stands for WHITE) leads to the turn signal switch.

[JonH]

A brake light switch simply completes a circuit. You should have 12 volts coming in on 1 of the wires. When the switch closes it completes the circuit sending 12 volts to the brake lights. Follow the wires, 1 needs to come from a power source, the other to the brake light circuit...

[anlauto]

Looks to me like the green wire was added and as mentioned, neither one should be grounded

[Dakota]

Based on the appearance of the connector, I'd agree that the green wire looks like it was an add-on.  Some guesswork:   I don't remember if your truck had a trailer hitch or not, but maybe the brake light switch got tapped to run trailer lights.   If so, you'll probably find some additional hacks on the wiring for the turn signal lights.

[Filthy Filbert]

Based on the appearance of the connector, I'd agree that the green wire looks like it was an add-on.  Some guesswork:   I don't remember if your truck had a trailer hitch or not, but maybe the brake light switch got tapped to run trailer lights.   If so, you'll probably find some additional hacks on the wiring for the turn signal lights.

Or a trailer brake controller.   It doesn't make sense to grab power from the pedal for the brake lights on the trailer.  most people would splice into the wires at the back of the truck and grab brake and turn signals from there. 

[MoparLeo]

I will recommend exactly what I recommend to all Mopar owners ,
GET A FACTORY SERVICE MANUAL. It will answer many questions that you will have as they have diagrams and servicing instructions for all systems.

[jimynick]

Based on the appearance of the connector, I'd agree that the green wire looks like it was an add-on.  Some guesswork:   I don't remember if your truck had a trailer hitch or not, but maybe the brake light switch got tapped to run trailer lights.   If so, you'll probably find some additional hacks on the wiring for the turn signal lights.

Or a trailer brake controller.   It doesn't make sense to grab power from the pedal for the brake lights on the trailer.  most people would splice into the wires at the back of the truck and grab brake and turn signals from there.

X2 for me. 

[blown motor]

I got them working. All good.

[jimynick]

So, you're out in your Taj Mahal gettin' wired, eh? Good on you buddy!