Originally Posted by acg_ssei

Sure: you probably have a bad ground in one of your brake light sockets. You might also have a bad (or wrong) bulb in one of those sockets, too.

What'* happening is that current on your brake light circuit is managing to double back through your taillamp circuit in search of a ground path. It illuminates everything along the way when it does that, though at lower brightness since the brake lamps and taillights are running in series when this happens. The only place on the car where brake-light and taillight circuits come together is at the bulb sockets, where they share a common ground. If the ground path fails, current in one circuit doubles back through the other.

Put your taillights on and get out and walk around. If one brake bulb is obviously shining brighter than the others, it'* either stuck in the socket backwards and shorting the two circuits together (only the brake filament lights up), or it'* a completely-wrong bulb altogether, such as a single-filament 1156 bulb whose center contact is shorting together the two socket base contacts.

If everything looks more or less normal, leave the taillights on and have an assistant step on the brake. Any taillight that goes out instead of getting brighter is your culprit, since both tail and brake circuits are now hot, and a bulb relying on one circuit to provide the ground path for the other will go out when both circuits are live at the same time. If the offending socket has a good bulb in it, then its ground connection is broken.

If none of the above pans out, get back to us.