what's wrong when brake lights won't go off on a 1992 Oldsmobile Cutlass Supreme?

Check under dash,it will be the brake light switch at top of break pedal,is bad.

Changed brake light switch and it still doing the same thing.

You must have a short in wire some where,check power to switch on both side,s ,if there is power to both switch wire,s you have a short.

Have power to both sides. Where could a short be?

Undo the brake light switch,see if light go,s out,if it dont start checking your wiring in trunk at tail light,s back to switch.If it go,s out when switch is undone,may be another new bad switch.

And one more thing to check,make sure you clipped the wiring harness in right at the switch.

When testing brake switch one wire should have power all the time and the other one should not have power until brake pedal is depressed then go out when pedal is released. If power going to both and not applying brake pedal make sure brake switch is installed correctly or is adjusted so that switch is making contact with brake pedal when it is depressed.