I have two doors on my 2007 CTS that won't lock or unlock with the key fob. In the past, the dealership has replaced the Lock Assembly, one time under warranty and the other time for $329. Do you think the lock assembly is the problem on these two doors as well? Or is there a fuse in the door lock that is out? How difficult is it to replace the lock assembly (any video available)? Thank you in advance for your assistance.