Do the light turn off after about 20 minutes? That is when the battery rundown protection kicks in. The Body Control Module controls the courtesy lights. I would suspect that one of the door inputs is telling the BCM to keep the lights on. Using a scan tool to read the date stream from the BCM may help in determining which input is keeping the lights on. I may be the case where each door switch will need to be disconnected one at a time to try and isolate the fault.