Tail/Brake light problem

I have a 2000 Grand Cherokee Laredo. The rear brake lights work fine in the daytime but as soon as I turn on the taillights (which work fine) and press the brake pedal both the brake and tails lights go off. The ony brake light that works is the one at the top of the liftgate. I checked the contacts and there is power to the bulbs for both tail and brake, the bulbs just won't light. I got new bulbs (3157) but no difference. I am totally baffled, please help.

Be sure the bulbs are seated propertly in the housing, and that they're the right bulb shape/size. Sometimes the bulbs fit twisted 180 degrees (the two posts reversed), and they'll work in one mode (such as running lights), but not the other (such as brake lights).

Are you getting power in all modes? (brake, running lights, turn signals, etc.). It sounds like a short circuit somewhere, which might be caused by a bulb being mis-aligned.

Check that the bulbs are seated properly and let us know if it makes a difference.