Guys,

Trying to help a neighbor out. He just bought a '95 Buick and when he turns on the headlights, they go out, as do the taillights. If he just uses the parking lights, both parking lights and taillights work. He has replaced the switch-no go. Fuses are OK. He says he has checked every ground he can find and cleaned same. Where should he look next? He says he has a wiring diagram but hasn't really tried to trace anything yet...so many wires in so little space! Any and all help will be greatly appreciated. I bragged a little about all the experts we have on this site and I know SOMEBODY can help.

Thanks so much!

Since he has the wiring diagram, use it. Get a voltmeter and follow the diagram until you find the problem. Newer cars are too complicated to start guessing. You can guess the rest of your life, and spend a pile of money replacing half the car and still not find the problem.
By 95, GM may have already been using the "body computer" that controls lighting functions. Pray it isn't causing the problem.
If you want to guess, junk the 95 and get a 65 where you can make accurate and cheap guesses.

The headlight switch powers a relay that powers the lights. On my parents 95 Le Sabre there were a bunch of relays under the hood along the firewall. They were covered by a 3' long plastic panel. Thier Buick developed electrical problems affecting the AC that neither I nor the dealership could figure out. I sold it for them and bought them a 2000 Le Sabre which so far has been a much more reliable car.

Do the brights work? Or no lights at all?