Why does the 3rd light stay on after turning the car off?

Is it the original or an aftermarket unit?

If original, the brake switch at the pedal assembly is the issue.

It's not all the brake lights; just the third light..... I'd first suspect it's a defective turn signal switch; then a blown fuse, or maybe a wiring problem, like a damaged wire.

It's only the 3rd break light. The light actually goes off after awhile. I thought it might be a ground problem. it is the original Does that light have its own circuitry since it's a led? Ed, if it might be a turn signal switch, then the turn signal will not work. Right?

When the lower lights do not come on and only the third-brake light works, the brake-light switch may be ruled out. Again try the emergency-flashers to check for wiring, socket and lamp problems. If the flashers work, check for blown fuses as a few vehicles use separate circuits for the lower and third brake lights..... On vehicles that use the same lights for brakes and turn signals, a bad turn-signal switch can cause the lower brake-lights not to work. With a bad turn-signal switch, the third-light should still function. On this type system, the turn signals and brakes use the same circuit. When we use the turn signal switch, it cancels the brake light signal, on the side selected. If it did not, the lights would stay on and not flash. A bad turn-signal switch may interrupt the brake light circuit when it should not. A broken wire to or from the switch may also cause the lower brake-lights not to work. The third brake light does not flash with turn signals. This is why a bad turn signal switch will not affect it.