Probably a few days late but for future info in case anyone searches: the switch that turns on your brake lights is the one closest to the passenger side of the vehicle. I know because over the weekend my brake lights wouldn't go out because the little plastic piece that actuates the switch and turns on the brake lights when the pedal is pushed disintegrated.
I'm assuming that you have done a full visual inspection of your exhaust system but the same exact thing happened to my pathy. The smell started while I was climbing the hills and I finally tracked it down to the failure of the final flange on the exhaust that least to the final muffler/resonator. The exhaust was leaking from a hole there and was shooting straight up to the rear floorboard and over a brake line. The brake fluid got cooked leaving me with a mushy brake pedal and a spot in the rear carpeting and matting melted leading to the horrid smell. Since the leak was after the main muffler there was no noticeable noise difference. I'd suggest getting under the truck and doing a thorough inspection.