This is NOT a fuse or power supply issue, if the control head is lighting up at all. Also, neither the vehicle's powertrain control module (drivetrain computer) nor body control module (body computer) is involved in HVAC airflow operation so neither of these components can be the source of your problem.
Your problem is most likely an internal problem in the control head assembly. To determine this for sure, a Vetronix Tech-2 scan tool will be required to check the operation of the control head and it's ability to communicate on the vehicle's data network.
It does not matter whether the system is manual or automatic control; the same problems occur with both versions. The affected components were used in production on vehicles built between mid October 2002 and January 2003. If your vehicle was built within this date range (date of build will be on a label inside the driver's door jamb area or on the drivers door) then yours has one of the control heads that has some serviceability issues. Part numbers of the affected control heads are 15202099, 15202100, 15202101 and 15202102.
These particular control assemblies can lock up and become non functional if they receive a voltage spike (such as when disconnecting or connecting a battery cable or by jump starting the vehicle). They are also not able to be programmed at a GM dealership with new software, or they lock up and stop operating (see GM technical service bulletin #03-01-38-003).
To determine for sure if the control head has become inoperative, you will need to take the truck in to either a GM dealership or to an independent shop that has a Tech-2 scan tool to have it checked. Replacing it with a new one also requires the Tech-2, to install the correct software calibration for your vehicle or it will not operate, so I am afraid this is one of those things you cannot fix yourself at home in the driveway.
I hope this is helpful; if so, please click my accept button... thanks!