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

[Duke George V]

Your defrost woes are something I can understand. Some time ago I called in to a local talk radio program that had some mechanics on and hit them with this: the third brake light doesn't work, the defrost switches on when I press the brake pedal, the cruise control doesn't work when the defrost is on. They were unable to offer any advice, instead saying I need an exorcist, not an electrician, because my shit is clearly haunted.

The 3rd light grounding through the tailgate sounds right. Both of my wagons have this same issue, though one doesn't have the cruise issue, and I think I know why. The defrost is painted onto the glass, and gets power from some copper plates glued to the glass on each side. If those plates disappear, you got problems. They go here, right and left:




And power goes through these prongs that touch them:




I think the previous owner of this wagon "solved" the problem by cutting the power supply to the third brake light. I think if those copper plates are replaced, it would cure the issue. It's not that big of a deal for me, since I live in Las Vegas, and it rarely gets cold enough to worry about defrosting the rear glass.