Rear Door Sensor

The door switch is likely the issue here. Try pushing on the door when it's closed and see if the light goes out, if it does the switch is loose or bad. You could have a wiring issue too, so check it carefully. I suggest checking the circuit with an ohmmeter. The switch should read about 0-2 ohms when closed, and infinity when open.

Just had a similar problem on my 96 A6 drivers side door. There is a small rubber nob that gets pushed in when the door is closed to communicate with the car that the door is actually closed. I had accidentally shut my seat belt in the door and snapped that switch. I ordered the switch from my mechanic and he replaced it in 5 minutes. cost me $15.