Hello, first time post. I promised my friend I would help him sort out some electrical issues he's been having with his van. 1990 G3500 5.7l. Have some experience with old Fiat wiring but not much with GM. Could all be related but here is the list:1) right headlight not working/works on high beam

2) reverse lights not working

3) front turn signals not working but hazards work, stays on when brake pedal is depressed
I know it could be many things...
Assuming all the fuses are tested good (I did a visual but will check with a test light) what would be the next step? Maybe there is a wiring diagram floating around that would be helpful? Many thanks, Mayuko

 

I have before looked at a fuse and thought it was fine. But once checked electrically it was not. Make sure the fuses are checked either with a test light or an ohm meter.
Do the same with bulbs. A burnt bulb can cause all sort of misbehavior like you describing. Specially flashers circuits.
Another suspect would be a bad ground. Check with an ohm meter ground side of the bulb socket to the body and see if there is resistance. It should not.

Any recent change to the van lately ? Shelf, flooring etc..

 

I might have wiring diagrams I can try to post later. You'll want to also check the plug at the back of the headlamp itself, the wiring can burn through in this location.