Center High Mount Lamp aka Third Brake Light inoperable

[MSPLS]

Trying to diagnose the third brake light in my 1996 Buick Century Wagon.

The first time a shop 'repaired' it, they said they removed a ground strap somewhere and it worked. Pretty hack. I found that resulted in the rear defroster turning on when hitting the brake pedal. 

Eventually it stopped working again, possibly when I replaced one of the rear lift struts that hold open the trunk. 
Another shop then fixed it by running a wire along that driver-side trunk prop-rod. Not sure where they tied into. It worked for a while, but again has stopped functioning.

I'm hoping someone here might have a high quality wiring diagram to share, so that I can DIY it and fix all the hackwork that has been done back there. 
I have these ones that I've attached, but they are so pixelated that I can't quite use them.

[73blazer]

I have the real GM Service manual for a 1989 A-Body. I can scan some pages when I get back there tomorrow. It'll have wiring diagrams and component and connector locations. If the 1988 is the same, I'd bet it its close if not the same.

[73blazer]

The stop lamp is grounded via the tailgate (which is a pretty crappy way to ground something, the hinges rust and no connection. I would be checking the ground as most likely culprit. Add a ground wire from a known good source to the light body.

Junction C301 is shared with the tail/stop and located just behind the LH rear wheel house, so if your normal brake lights work there's power to them here and the high mount stop lamp is shared off that feed. So your problem, if it's not the ground, is between C301 and the actual light.

Again this is from a 1989 Pontiac A-Body service manual. 1996 mabey be different, but I doubt it!


   
   
   
   

[MSPLS]

The stop lamp is grounded via the tailgate (which is a pretty crappy way to ground something, the hinges rust and no connection. I would be checking the ground as most likely culprit. Add a ground wire from a known good source to the light body.

Junction C301 is shared with the tail/stop and located just behind the LH rear wheel house, so if your normal brake lights work there's power to them here and the high mount stop lamp is shared off that feed. So your problem, if it's not the ground, is between C301 and the actual light.

Again this is from a 1989 Pontiac A-Body service manual. 1996 mabey be different, but I doubt it!

It was just a bulb that went out x.x
At least I now have a better quality diagram to work with. Maybe I can figure out the rear defroster issues next. Thank you