AS you see I have a 97 and live in the Mid-West and don't notice to many third eyes working. Today received my Dorman part #923-201. And it was easy to replace and works like a charm.

 

1995 GMC Jimmy SLT 4wd .
Third brake light above Rear Hatch Door.
When my third light was out I picked up a replacement at Junk Yard.
While repairing it, I found that the wiring which runs from the rear cargo area dome light opening , up to the Third Brake Light, was open circuited. The connector inside dome light housing hole to the external Third Brake Light LED bar.

It appears GM did not strain relief this wire well ( per design) and it is also subject to outside weather.
Every time back hatch is opened closed this wire is in flexing mode !
I corrected and strain relieved using some "gasket maker RTV" .
This open circuit at the latter part if the wiring may be why you don't see many Third Brake Lights operating !

 

Glad to see this thread. My 3rd eye is intermittent; now I know where to look!

 

This 3rd Light is so so exposed to the elements !!!
Found three hairline cracks in the plastic housing on top portion.
Right securing screw seat of the plastic housing broke its screw seating ability.
Repaired by replacing both securing screws with new 4.7 mm screws , ( 2 ) 5/8 in. rubber/metal washers.
Hairline plastic cracks sealed with black RTV.

 

If the third brake light isn't working, will the brake lights below not work? I have no third light and no brake lights.

 

they are not connected.. here are some wiring diagrams for the third brake lamp and for the regular brake lamps. hopefully that will help you track the issue down

 

Thanks for the wiring diagrams. They are very helpful. I am back to the 'Multifunction switch' as the probable culprit for the 'no brake lights, but have tail lights' problem.