87 5kCSTQ Mysterious Brake Light

[Paul Meyers]

So, during the year storage in the back yard, I bashed a quarter-sized hole
in the right rear taillight assembly on the Gray 87. Chris Semple sends me a
new taillight (thanks, Chris). I install it last night. Easy to do with deep
8mm socket. Hardest job is reworking the sticky black goo used to keep water
out. I go for a test drive and on comes the warning light telling me my
brake lamp is kaput. I go around to the rear of the car, having son press
brake pedal. Sure enough, the brake light no longer lights. I know it
worked, since the car just passed the state safety inspection. So I figure
the light bulb must have coincidentally burnt out. No, filament still good.
I check all the fuses, Nope. 

Then I turn on the headlights. Now the mysterious behavior. The brake light
now comes on with the taillight! Other brake lights work normally, but this
one now only comes on with taillight and fails to work as a brake light. 

No connectors were removed or reattached in the lens replacement process.
The bulb carrier just snaps out then snaps back into the new lens assembly.
There is no electrical contact between the bulb carrier and the lens
assembly, so it can't be a ground problem.

What gives?

Paul Meyers 
Gray 87 5KCSTQ 1.8bar eurolights fuchs 147 kM
Pearl 88 5KCSTQ 1.8bar eurolights fuchs bilsteins 128 kM 2pc-em k24 turbo 
Repainted Pearl 87 5KTQA 1.8bar eurolights TT wheels bilsteins 203 kM daily
driver
Blue 86 5KCSTQ parts car