We have a 1993 3500HD gmc,the brake lights aren't working, we have replaced the bulbs and replaced fuses. not sure what else it could be can suggest some ideas.

The tail light fuse is under the rear seat on the drivers side.

check your Head Light Fuses I know for some reason that Happen to me and I had to replace my head light fuses. I would also check the circurt line out and make sure its getting power to it.

seems you have several different problems. High beam working and no low beams=dimmer switch is bad. Reverse lamp not working=bulb socket bad,wiring broke,bad ground at back
Brake light on drivers side stuck on=brake light switch

Check the fuse but it is possible that a bulb can be out

i have a 1995 gmc truck backup lights not working

check for a bad brake light fuse! if not the fuse, the brake light switch operates brake lights and if bad you will not be able to shift it out of park, as well as the brake lights not coming on.! because you have to put foot on brake to do so. if bad switch, it wont let it out of park. it could be just out of adjustment also!! good-day !

Brake light switch. If I remember correctly, the third brake light works off a seperate circuit from the other brake lights so part of the switch could be bad.

The high mount brake light has its own fuse, so check that first. The high mount bulb is generally a stabdard type (there may be more than one bulb) or you can replace with an LED. As for the left rear bulbs not working when depressing brake pedal, check the bulbs again for any spring that may be broken. If the bulbs are still good, make sure there is sufficient contact with the socket. If still not working, have someone check it out further.

Make sure that ypur parking brake is completely released because if it is engaged even slightly the daytime lights will trun off. If that's not the problem then check the bulbs.

One of two things cause this corroded or poor chassis earth at the bulb holders or the circuit board that holds the bulb is faulty.