i have a 1992 gmc sierra k1500 , when my lights are on , my brake lights stay on constantly. If head lights are not on, brake lights work as they should. Any ideas on where i should look ?

When your headlights are on in the front, the running lights are on in the back. It could be that the bulbs are in the sockets backwards. It could be that the power is backfeeding through the brake light filaments. --- Remove the bulbs and check what wires have power with the headlights on and off and the brakes on. Check the ground connection at the sockets too.

What kind of environment is this truck stored in? mostly wet most of the time? ageing UV and sunlight environment. Extreme thermal cycling...zero at night to triple digit days? environmental concerns are everything!

Try a couple of new bulbs in the back I think the filaments are touching inside one of your rear bulbs. I have seen this before. Change one then see if it's fixed you might get lucky and hit the offender on first one.

Where should you look ? The schematic. You can obtain one from autozone website because these trucks had so many variations. understand your own before spending money on parts. The wires, and which lights do what, when can be deduced by studying the correct schematic. What components are actually in use on yours, where are they located, what circuit protections are in use, where are they? The circuit boards and the grounds. the proper installation of bulb parts. It will all be difficult because of having so many rewires for different years so get the right map to your destination.