I'm having a problem with my 3rd brake light. Only 1 of the sockets has power and the other 4 do not. The metal strips inside look ok but I must be missing something. Any help appreciated.

If the metal strips look ok, it is probably just some oxidized/dirty contacts in the bulb sockets themselves. You can test along the metal strips to ensure you are getting power along the whole thing, then just try to clean the bulb contacts. However, cleaning them will probably be a pain 'cause they're so small. I would try taking a small nail or something (maybe a tiny drill bit or needle) and just sticking it in the sockets and scraping around with it... Make sure whatever you use is smaller than the bulb leads, though, cause you don't want to enlarge the sockets at all.

'bird or 'maro? where is the light located? depending on the year (and sometimes model) the third brake lights can be completely different.

For the record I was assuming the brake light is the same-ish as mine ('89 Camaro): 5-bulb, spoiler mounted.

Ya its an 89 camaro spoiler mount. I'll try to clean the contacts tomorrow. Thanks for your help.

had a similar issue but it was on my 86 IROC-Z. My 3rd brake light is outside and high up on the rear glass. My cousin who was a Black Hawk mechanic for the US Army checked it out. He ran a battery of tests on the strip of metal that housed the 4 light bulbs. Results were that the metal was structuraly non conductive. Even though the metal seemed to look flawless, it was no good. so I went to the junkyard and took off one from a Trans Am. Sure enough the Trans Am one worked. The same issue might be occuring with yours.