I have a problem with my headlights, the side markers on the driver’s side do not go on with the headlights. The turn signal indicators on the dash are lighted and stay on constantly when the headlight switch is on. I believe this is caused by a ground problem. Any help or ideas will be much appreciated.

1970 mustang fastback

 

I have a 70 Mach 1 so forgive me if I forget the light arrangement on the 70 fastback. On my Mach 1 the lower turn signal lights (under the bumper) need to be bolted to the bumper. This connection provides the ground for a lot of the front end lights. Also, the high beam filament inside your headlights acts as a ground at some switch configuration. I spent a week trying to get my lights to work, (my symptoms were just like yours). I read the schematics a 100 times before I suddenly realized that I needed to have my bumper on the car with the lamp housings bolted on. I too had constant signal lights on the dash (they were dim) and no front turn signals when the headlight switch was engaged. I know the fastback doesn't have the sportlamps in the grill, but I believe you have the under bumper turn/running lights)amber). Hope this hekps.

BTW..you are correct in believing it is a ground problem.

 

That’s just the information I needed, my front bumper is off and one high beam is out. Thanks so much I have been restoring this fastback for two years now and I couldn’t remember all the glitches. I’m sure you have hit the problem.
Thanks again