if you need to do more in depth troubleshooting then a tech2 is the tool you need, they run about $300 for a clone but it pays for it's self, same tool the dealer uses.
the gmt900's use a lot of high speed lan communication, -vs- the older platform which also used a lan but not nearly as much, the door switches are basically modules a digital signal is sent to the door switch and the chip inside decides what to do from there, for instance if you have blinker mirrors the blinker function comes from the door switch instead of a traditional signal wire from a relay, it comes from the bcm but it is digital to the door switch, same for the lock actuator. with a tech2 you could send the signal and then determine where the problem is, wiring, switch, actuator, bcm, etc the screen on the tech2 tells you if the signal is being received or not.