Searching other threads has not helped.
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I drive an '86 Firebird. I was in an accident last month involving my front, driver side (will replace bumper, headlight, fender in a couple weeks ). Since then my brake lights do not work, EXCEPT for the 3rd brake light mounted high and center on the hatch.
All bulbs have been replaced recently and appear in good condition. My tail lights work when my headlights are on or when my parking lights are on.
My front fender turn signal does not currently have a bulb ('cause of the damage), but does not fix my problem when I put a bulb in it.
My break switch is new and shouldn't be the problem since the 3rd brake light works.
I have duct taped and wired a temporary headlight into place.

What might cause my brake lights to not work, while the tail lights do work?

I wouldn't discount your brake light switch yet. I haven't screwed with the brake light circuit on the thirdgens very much, but on my Corvette I had exactly the opposite problem. My brake lights worked, but the 3rd brake light didn't. GM uses a brake switch with two separate outputs... one for the 3rd brake light and one for the other lights. It's quite possible that the switch is failing. When I tested mine out of the car, it would seem to work just fine, but when it was plugged in and had current passing through it, it only worked once in a while, so make sure you test it with the harness hooked to it, and not just with a digital multimeter. Or for the $7 it costs... just replace it.

If that doesn't work, keep in mind that the 3rd brake light is still a separate circuit from the main circuit, and that should help you in your diagnostics. It's possible that it's something as simple as a fuse.

Ok. I just don't know anything anymore. It was the brake light switch. Dang. And I just replaced it last summer. Lucky I had an extra to swap into the car...lucky my dad was a mechanic for many years and had a hunch and bought me a switch weeks ago!

I just didn't realize the switch controlled the main lights and 3rd brake light separately.

I'm a dork.

I didn't realize it until I had problems with my Corvette. I was pulling my hair out until I grabbed a test light and started fooling with the switch.