It probably wouldn't be a bad idea for me to get into the driver's side door to see what's happening in there re: the latch and sensor.

I read about testing the Hall Effect sensor; I've got a good digital voltmeter, so I should be able to accomplish this. Is there any easier way of testing these sensors (besides pulling the door panel off to access the latch, etc)? I guess I could measure mA at the GM, but I don't have wiring diagram.

Probably time to invest in a service manual . Or would the dealership be able to easily diagnose (and fix) the issue?

Thanks Jeff!
Dan