Took the car in for annual inspection and center high mounted stop light didn't work. Shop guys (of course) immediately diagnosed problem as burned out led light costing upwards of $100 to replace and began to tear into it. Of course, no leds in an 86 IROC CHMSL so they pronounced it a blown fuse. Since stoplights in the back worked that wasn't it either. Once guy took the lights out of the CHMSL, put the lense/cover back on it and pronounced that it was now an "ornament" and not a light. He passed the car but made the other shop guy mad.

Now for the real answer. The screw that connects the light assembly to the ground wire (inside right side of hatch glass) was loose and corrosion had built up between the copper (coated??) washers, the screw and the light assembly. A little bit of sandpaper took care of the corrosion and after reinstalling all the parts (and replacing the fuse the shop guys somehow blew) everything worked again.

A little bit of trivia is that I didn't know the fuse was blown and drove it to work the next day with my shiny new inspection sticker in the window. I noticed when I got to work that when I turned the ignition key off with the headlights still on the warning buzzer failed to tell me I was about to run my battery down. A friend pointed out the headlights were still on. While checking the owner's manual it pointed me towards the fuse that supplies power to the warning buzzer but it was okay. When I replaced the brake light fuse the warning buzzer began to work again. Hope this helps someone else out.

 

Another odd manifestation of problems with the 1986 model year hatch wiring is that if one of the ground connections has failed, and the vehiocle is equipped with a rear defogger, application of the brake and lighting the CHMSL will back-feed the relay for the defogger and cause it to turn on. If you're driving along and notice that the defogger is on and you didn't turn it on, check the ground connections for the rear hatch. Start with the spade terminals on the hatch gas-charged struts.