Brake lights on a 2003 chevy silverado 2500HD wont work.

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Did you check the park fuses, note the diagram, rr and lr. Use a test light to check voltage at fuse circuit, headlamp switch on. That should tell us if problem is before the fuse or after.
You always want to check voltage and ground circuits, for lamps. Do the front park lamps work?

Brake switch is on the brake peddle arm, look up under the dash, held on with a clip, remove clip and slide switch off.

heated seat eliment needs replaced. or unplog

check brake lights fuse.if fuse is okay.brake lights switch could be faulty.

Unless you really know what you are doing it is best to get a mechanic to repair this. People's lives and safety are at risk and you could be held legally if something went wrong with your repair job. I would not touch it.

See if the turn signals work,and the flashers,the problem,is most likely in one of the flashers,they are stuck,the flasher.

If you have an aftermarket trailer towing plug on it, that's where i would look first. Something is shorting out in there, or you have a broken wire or ground somewhere. Proabably near the back where gravel stones can hit.

you need to change your brakes. they have squeaker tabs on them to let you know they are worn real low.

check fuse's, if that doesn't work check bulbs

your truck has fuses for the rear and high mount brake light in the fuse box in the side of the dash when you open the drivers door