probably the brake light switch on the pedal if none are working at all
If none of the brake lights are working i would suspect the brake switch at the brake pedal.
Could be the brake light switch or fuse.
First check your fuses if none are blown then check the switch.
If NONE of the brake lights are working (including the "high mount" center light, I'd check the stop light switch next. It is located up under the dashboard near the top of the brake pedal lever. CHeck to see that it is still in proper adjustment (barely touching the lever, and if it is, it may need replacement.
Check the plug assembly under the back of the truck near the hitch frame. All of the lighting wires run through it, and a loose connection or loose plug pin can cut all the rear lights off (been there, 1995 suburban).
If the fuses are all good (which, I bet they are), you most likely have a faulty dimmer control. Almost all the dash and interior trim lights are wired through this switch to allow you to dim them. If it has gone bad, that could cause them all to stop working. Fairly inexpensive and easy to change. Check the fuses-- under the dash and/or under the hood.
the side lights would be fusedseparately so either if none of them are working and the fuses are ok it sounds like a faulty switch.but you would have to check for a 12v supply to the lights with a muilty meter to check it out properly
well if your head lights and tail lights are still working then its your dimmer switch that has gone bad and needs to be replaced. i just recently had this problem and was fixed with a 10 replacement off ebay.
you need to check your fuses first ( sounds like it) they are on underdash (drivers side)
im not sure but sometimes there are relays under the hood; maybe one of those are bad.
Check fuses (if stop lights are not working shift interlock will not release) Check fuse box for corrosion or loose fuses