Right Rear Brake Light Works, Right Rear Turn Signal Doesn't?

I have a 2003 Buick Century. The rear lights are dual-filament bulbs (low filament for running lights, high filament for turn signal/brakes). I noticed that the turn signal wasn't working on the right side, so I replaced the bulb. No good. In poking around, I figured out that the brake light was working, so the high filament was getting electricity through the brake circuit, but not through the turn signal circuit. What's the problem? Bad relay?