I discovered I had no brake lights, or hazards, the other day. Running lights work fine. I replaced the fuse and as soon as I hit the pedal it popped. I tried it again, but only the hazards and it didn't pop, until I hit the brake pedal. Could this be the brake light switch? Do they short out like that when they go bad? I previously had been having trouble with the brake lights staying on when I wasn't pressing the pedal.
Any trailer wiring on your car?
Probably not the switch - you've got a short to ground somewhere.
Things are crossed up I bet. The worst layout I have seen thus far on mk1 cars is the truck. The units for the bulbs in the back can do very wierd things when the ground goes crazy. I have been fixing up an 82 and as soon as I applied battery to it, only a couple things were working! It was freaking me out at first because the truck was so clean with no cuts in wiring but I worked them out one at a time.
I ended up having to drill out those housings in the back and running fresh ground leads, etc. Mine weren't even really dirty but I had crazy things happening like you. Brake turning on/off things. Different brightness in one side vs. the other, etc.
So I would make sure there are no cuts in factory wiring from wierd old owners and work yourself with a meter through the legs. And of course make sure you start with sound power/grounds in the engine bay to chasis. I always end up making my own cables and starting fresh with cleaned and extra grounds.
It does have trailer wiring, but it was working fine, brake lights worked and everything. I ran the wire in conduit so nothing should rub through, but it looks like I'll have to check it all out again. My main concern was if the switch could cause it, but sounds like it isn't. I wasn't looking forward to tearing into the wiring.
iirc, the switch is just a simple two wire deal possibly powering a relay. The wires are awful small gauge, so i cant see it carrying amperage.
What all have you done to the truck before it started to act up on you?
damac covered the most likely problem; ground wires, check and clean them up.