I have 1999 F150 ~255K miles. The brake light on the back of the cab is working backwards. Lights on, the brake light is on, push the brake pedal it goes off. I replaced the entire assembly, same thing. Looking of suggestions on how to diagnose. No work has been done on the light or the wiring harness (original owner).

All the other brake lights, turn signals work as they should so I'm thinking it's somewhere between the wiring harness in the back of the cab, back to the source. I've looked for a wiring schematic but can't find one. Are there switches / relays/fuses I should check? Again, all other brake lights and turn signals are working fine.

Appreciate any guidance.

GM

 

That sounds like a bad ground.

 

Does your roof have a leak in that area? Apparently they are notorious for leaks

 

Not that I have noticed.

 

Yep, open or high resistance ground circuit. Remove the front passenger side kick panel and inspect the grounding studs and wires. Look for body rot or corrosion. If no problem found there, check continuity of the ground wire from the lighting assembly back to ground looking for a wiring fault.

 

You mention remove the front passenger side kick panel. Is that where the wiring runs and is grounded?

Now that I think about it, I did have a leak around the windshield near that top passenger side corner where the window meets the top of the cab. Like thee rubber molding seal around the window was leaking there.

 

Yes, hence the reason for the instructions I gave.

 

OK thanks, I'l check that out,