Repairing a faulty temperature blend door is no easy task. Although the replacement part only costs about $35-40, the extensive labor required makes the a service center repair cost around $1,100-1,400. The service center I visited quoted 11 hours from start to finish for the repair. An author on another site showing a step-by-step on the repair writes that it takes 24 hours to complete. A few friends offered to help me with the repair, but I preferred the service center route since I got GM to pay half.
If you want to do it yourself, here's the step-by-step I mentioned at Handymanlyness: