Weird Electrical Issue...

[anlauto]

My signal lights all around work fine, as do the emergency flashers.
My park lamps work fine all around, on the first "click" of the headlight switch as well and with the headlights on (second click)

When I have the signal lights on, flashing either left or right, they work fine UNTIL I turn the headlight switch on, either click...THEN the front (only the front) signal light stop flashing and just stay on bright with both elements of the 1157A(incandescent NOT LED) bulb lite up

If I un-plug the park lamp connector and turn it 90* and plug in only the signal light half.....Of course I have no park lamps come on, but the signals work fine with or without the headlights on. 

[toadstool]

Guess... Either the lamp bulb is not fully rotated in the lamp socket and the two lamp pins are shorting to each other to the lamp socket. Or grounding

[Filthy Filbert]

Fix your grounds.

It sounds like the light is, instead of grounding normally, it's backfeeding through the positive side of the opposite light being commanded.   Once you command BOTH turn signal and parking lamp filaments, you have nowhere for the ground to go, and you end up with a 0 voltage condition..