inside the door there is a switch that activates the interior light when the handle is tugged.
I had a problem with mine after I put in an viper alarm system kids would walk down the street and flip the door handles of all the cars on the street. Very upsetting at 3:00 am. GRRRRR
Anyway, Below the dash on the left side of the Steering when. You will have to remove the black dash cover above the Break peddle. Now lets see if I can explain this right. If you put your head below the steering wheel and look at the OBD2 connector. You will see between you and it the factory colored Dash piece. If you put your hand up behind it and on the back side of it, almost right in front of the ODB2 connector you will find a small black box. Measures about 2 inches by 3 inches and an inch high. there will be (by memory) about 8 wires coming out of it. There are no markings on the box to tell you what it is except a GM 1708XXXX number. Unplug this box from the wiring harness and check to see if the light is now out when you close the door. HTH