I have an 04 Envoy XL. I have a problem with the rear a/c. When I control the rear a/c from the front, things work fine. I get cool air in the back. I switch the front switch to "R" and the rear control is set to cool, the air temp then warms up. Switch back to control from the front and the temp is cool again. I'm thinking the temp actuator in the rear. Any other ideas? How can I test it?