I think the door sensors is a good place to start. The door lights and interior lights run through the RFA remote function actuator and the BCM body control module. There is a test that can be done to determine if either of those is bad as well as the door sensors.
I don't think it'* listed and hope someone with their manual handy corrects me. I believe with the ignition on you press lock and hold, then pull the headlights on and off 6 times. Then each test you do like lifting a door handle has a response from teh system. (off to search for it)
00 Regal GS: All kinds of cool stuff.