It sounds likely the heater / AC computer was damaged by the old generator. Due to their design, that is a fairly common problem. At the risk of locking up the radio or other computers on the truck, you can try disconnecting the negative battery cable for a couple of minutes. In some cases that will reset a confused computer. I don't like to recommend that because the GM engineers have devised a number of tricks that require you to take the vehicle to the dealer. Simply disconnecting the battery can lock up some computers and the dealer has to unlock them. They started that nonsense on some 2003 models.
You can also check for a leaking or disconnected vacuum hose under the hood. Some heater control systems use vacuum motors to run the temperature and mode doors. Usually they are spring-loaded to default to heat / defrost when there's a failure so for safety the windshield will be clear.
Monday, June 17th, 2013 AT 10:25 AM