[Ryengoth]

Sounds like the diode box "filament monitor" in the back is faulty. The only way to know, for sure, is to follow the service manual and check the voltages coming in and going out of it. I have a spare unit that works. I replaced mine and it turned out being the brake switch.

 

[kflynn]

My apologies for the delay. Do you know how to check the condition of the filament monitor?

Thanks

 

[Ryengoth]

9000 Service Manuals

You'll have to look in the electrical section for the wiring diagram for your trim level. There are some step-by-step diagnostics for the brake lights, as I found digging through for my own issues, in there but I don't recall what PDF or where it was. Please grab what you need for future reference since there is no guarantee they'll be up forever.

 

[davide c]

Many ! tailight /brake light issues on 9000's have been down to Bad grounds .. often with resultant meltings in the Light bulb holders in the tailights.
Pop 'em out and look closely. Then hunt down the trunk mounted Ground point.
G'luck