I have an 86 IROC-Z and I tried to take it through inspection. They failed me for not having a third brake light installed. I can't find any wiring or mount anywhere to install one! I know my 86 Z28 had one on the inside up top of the back window, but there were mountings and wiring that doesn't exist on my IROC. I could get one of the stick-on ones, but if it didn't come with one, how would I install an original one?

 

The '86 3rd brake lights were mounted on the outside, on the top of the hatch. Your Z28 may have had a hatch off a 91-92. Anyway, as far as wiring: You could just splice the wires into the existing brake lights. But, if you do that it would blink along with the turnsignals. The proper way to do it is to run the wires to the brake switch just like your existing lights. I don't know the wire colors for your car. Get a chiltons manual or something that has wiring diagrams.