RepairPal Certified shops can diagnose and repair your 2006 GMC Savana 3500. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
If it is a blend door issue, you will probably have to change the faulty actuator. This is the heater blend door actuator for your Savana
http://www.autozone.com/1/products/68452-a-c-heater-blend-door-actuator-dorman-604-106.html
how do you fix it