The modules go bad sometimes. And the wiring harness where it flexes in the trunk.
But start with the bulbs like he said, that's the easiest thing.
Make sure it's the right bulb. And the same as the others.
Those systems can be more finicky than an insecure pregnant wife....
*Should probably explain what that is, huh.
Your car has a circuit that checks for a difference (can't remember if it's current or voltage) between the rear bulbs. If it sees a difference it turns that light on. It's supposed to mean you have a rear bulb out. But the cars are so old now other parts of the circuit malfunction a lot of the time. So start by checking your rear bulbs. Hopefully one is burned out and it's an easy fix. If not, check the stuff I mentioned. Start by making sure all the bulbs are the same and there's no corrision.*