No brake light but third brake light still works...wtf

Right now my daily driver is down and I need to figure it out ASAP.
The truck is a 1995 GMC 1500 2wd SCSB 350 and a nv3500.
I was driving to school a few days ago when someone pulled up next to me and told me my brake light weren't working. I thought ok no problem probably just a switch on the pedal needing replaced. I pulled it into the shop at school and noticed my third brake was working but not the main 2. So I know the switch is still good. I checked my flashers and they still work fine so I know it's not a broken wire to the bulb and I know the bulbs are still good. I checked the relay and it's working fine. I changed it out just to be sure and still nothing. I thought maybe it was a ground so I redid the grounds at the firewall,chassis, and engine block. I also redid the grounds at the rear of the frame for the lights them selves although since the hazards are working I don't think that was an issue. I ended up checking it the next day and messing around with it for a while. I accidentally left my headlights on for about 20 minutes while I was doing this. I pulled the dash apart and pulled the headlight switch out after someone told me it could be in there. After the light being left on for only 20 minutes the back of the switch was so hot it almost burned me when I grabbed it. Since I have aftermarket headlights and high output bulbs I thought they were just over loading the switch. All the wiring looks great so I replaced the entire headlight switch assembly with a brand new one. It's working great but nothing has changed with the brake lights. Still nothing.
I'm not sure what else to do short of running a wire from the third brake light through a relay to the main lights. I really don't want to goober it up like that. I'd like to fix it the right way and be done with it. I just can't figure it out and don't have the extra cash to be blowing on parts I don't need.
Any suggestions??

Sorry for the long post just trying to give everyone the whole picture.

I almost forgot to mention that all the fuses and relays have been replaced. Also the circuit boards and bulbs in both tail lights are new.

ok well you have 1 of 2 issues. but first check things what lights do you have back there. do you have tail lights, do you have turn signals. see if your hazards work. now if they all work but no brakes have someone push the brakes and test again. do you have trailer wiring set up? look at that make sure you dont have a ground out.

I had this same issue on a 95 work truck, it is a relay under the hood, third brake light worked but no other brake lights, for some reason it would need replacing about every 6 months, if you post a
Picture of the relay box I can probably tell you which one

Multifunction switch will cause this issue. My 99 blazer had the exact same thing and was actually recalled. Not sure if it happens on these trucks or not. Might be worth swapping one out and trying

And put a relay kit on your headlights before you burn up the harness and switch...the replacement pigtail for the headlight switch is like 70 f'kin dollars.

Richard

Yup burned through a couple of them too

I don't have any trailer wiring and all lights are working except the brake lights. All the wiring looks great.
As for the relay I've changed all the relays and fuses already. I ever pulled the brake light relay and used a jumper wire to jump the constant 12 volt over to the brake light load terminal with no luck. I also checked the ground at the relays and it's all good.
I have also heard it could be the multi switch but is there a way to test that assembly?? I don't have the cash to drop $200 on a new one for it to not work. Rock auto has them for $100 but locally their double that.

On my headlights I figured I needed a larger relay. Every now and again when I run them they shut off for about 1 minute and than come back on. I figured it was the relay getting hot and shutting down than comig back on once it had cooled down. I looked and looked for a head light relay and haven't been able to find one. Does this truck have a normal relay or is it some thing inline?? I need to just order a set of the bigger relays and be done with it. I think the relay kit for the headlights on these trucks is only about $20.

Dunno on your brake light issue but on the headlights, changing the relay isn't going to solve the issue, the relay is just for the high beams IIRC but regardless the full load of the circuit goes through the headlight switch which is a shitty design.

For like $14 you can get a relay harness pre-made so all you have to do is plug it in. I ordered mine from DDM Tuning when I got my HID kits.

Richard

I dont know how to test the multifunction switch, i would get one from a bone yard. I would imagine you could poke around with a multimeter and see what you find. On my blazer it had something to do with the hazzard part of the switch. Maybe try and see if you hazzards work? They shouldnt have anything to do with the 3rd BL so may be a sign if they dont work...

On my headlight switches it was always the running light wire that would melt down. Never happened until I would tow a trailer with lots of marker lights. I put a relay in the left tailight cavity that would use the factory trailer wire for running lights as the trigger and ran a wire from the relay to the trailer plug so the trailer running lights were running only off the relay. Never burned one after that.

Try pressing on the hazard switch but not hard enough to actually turn them on. then while doing that press the brakes to see if they work( with the help of someone watching) Sometimes this will work in finding if the mfs is bad