Ok, it's not exactly a home-related issue, but I also know that some of the people hanging out here are savvy enough about all things electrical to give me a clue.

Here's the problem. I've got a 2000 Jeep Grand Cherokee and I had another driver tell me my left brake light was out. The brake lamp function and the tail lamp function share the same bulb. When the car lights are on, both left and right tail lamps are lit, but, when I step on the brakes, not only does the left brake lamp not come on, but the left tail light goes out, too; when I take my foot off the brake, the tail light goes back on. I put in a new bulb, but the same thing happened.

Can someone help me figure out what is going on here?