GMC Envoy HAC Blower problems

My Envoy Air Conditioner lost power on the high #5 setting first. Then it would blow and cool on some days on the other settings, then it would blow and then just stop. The next day it would work fine, the next not blow. Now the heater does the same thing. When driving on the highway, though you do feel cool or heat whatever you have it set on, coming through the vents. One night last week, I started it and the blower was working, warming the car.????????? Any suggestions where to start on repair of my problem.

Sounds to me like the blower speed switch is faulty. You don't mention a model year, so if it is less than 3 years old, it should be covered under the bumper-to-bumper warranty. Otherwise, you can buy new from the dealer or search for one online or at the used parts yards. Good luck!