If Zs suggestion doesn't work, it's easy to test for power at the bulb end of things. Just pull the bulb and you will see 2 terminals in there... one is for the taillight, the other is for the brakelight. Ground the negative terminal on your tester to where the bulb housing makes contact, and touch the two terminals one at a time with the positive terminal (make sure a brake is engaged for this test). If you get a reading on both terminals the circuit is okay.
While here, might I suggest throwing one of these in your toolbox. They are inexpensive and are PERFECT for testing circuits!
