74stepside - Hey, I just want to thank u for sharing that link to the diagram of the lights. I had the same question, but my brake lights wasn't turning on when I turn on my headlights in the evening (got pulled over for it). I took a look at the fuse panel under the steering wheel - replaced the fuses. However, when I insert the BL (brake light) fuse, the light underneath dashboard would come on. Yet it would blow out the fuse immediately when I turn on my headlights to check my brake light. I don't know what the deal is with this, but I'll have to follow your diagram to see what the culprit is. Any other insights would be helpful. The lights were working fine before and I assumed it was a blown fuse, but replacing the fuse proved otherwise.
Thanks again!
Proud Owner of a 86 Chevy 4x4!! =)