Remove the climate control panel and check it over visually for burn spots or excessive corrosion. An A/C specialist can tell you if the panel functions. The solenoid for plenium actuation may need to be replaced if the control panel is not at fault. This can be done by the home mechanic but most of the dash will need to be removed to provide access. An A/C specialist can save you alot of frustration in the process and it may not be as expensive as you thought. All of these parts you will be dealing with are fairly expensive ($250 for a climate control panel and $100 for the solenoid) and can only be purchased through a GM dealership.