Hello & Welcome,
Your problem is most likely an auxiliary actuator .here is some info to help you replace it.
Rear Quarter Trim Panel Replacement - Right Side (TrailBlazer EXT, Envoy XL)
Auxiliary Temperature Valve Actuator Replacement
Notice: Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.
Tighten Tighten the screws to 1.9 N·m (17 lb in).