Posting for a friend, were stumped.

87 F super duty, zf 5 speed, 460, 10yd dump body with aftermarket running lights and 18 wheeler type brake light setup (large standard issue lights, nothing special). I stress the cab is using ford factory lights, but the rest is all aftermarket. No idea on the wiring.

The turn signals will work as do the brake lights and running lights IF the headlights arent on. Turn the headlights on and press the brakes and for some reason (never did it before) running lights work like the brake lights-press it and it blinks on and off. brake lights do not work when headlights on, blinkers do for some reason iirc (its really complicated) UNTIL you press the brakes.

While foot is on the brake you put the turn signal on sometimes the opposite side will blink or the blinkers wont come on and light faintly on the dash.

Turn the headlights off, everything works as should, turn just the running lights on everythings fine.

Bad ground in headlights? Checked all fuses they are good, replaced flasher cause it was weak (clicks strongly now), hazards work. I dont get it. Everything worked fine, dont use the truck too often tho.

 

No one has any ideas?

 

Bad ground in tail light wireing useing running lights for groung.

 

The dimmer function in the head light switch, does it work correctly?

Test it with lights "on" but no headlights, see if the dash lights go full bright to dim.

If not I'd suspect it's rheostat, coil wrapped around base of switch, as the cause of the problem.