I have no brake lights on my 98.5. I have turn signals, flashers, parking, running lights. I also have the top rear cab brake light and it works fine.
I have changed fuses and bulbs. change both the 40 amp trailer fuse and 20 amp stop light fuse. they were good and are still good. it seems as though the turn signal uses the same wire and filament as the brake light and the turn signal works fine.

any idea's ?

 

I would have said brake lite sw but the cab brake lite works, I would try a different flasher unit for the turn signals then try the hazard lite sw to see if smooth, if not try cleaning it.

 

the flashers and turn signals work fine. can it still effect the brake lights when the flashers and signals work?

 

I would check bulbs replace with known good - sometimes bulbs look good but contacts are bad), bulb sockets & grounds then trace the wires forward along the frame.

 

My thinking was.. what over rides the brake lights? Hazard warning lights and turn signal on the one side...I haven't had a chance to look at WD's but I'm thinking the hazards aren't retuning to normal, switch or hazard flasher.

 

may be the turn signal switch, power in, none coming out. dealer only item so I'll look next week.

 

Check the brake pedal switch under the dash. But if your single upper brake light works then its probably the bulbs.

 

I had already changed the bulbs, and I replaced the brake switch with one from my 96 truck. I think it is the turn signal switch but will follow up on that next week.