My wife has a 2012 suburban. We noticed this summer the rear AC would blow slightly cold air and room temp air back and forth a few seconds at a time. Now this winter we are noticing that the heat is doing the same, slightly warm air to cold room temp air back and forth. Any ideas before I hit the dealer ??

 

Update, I took the trim pieces off the rear heater to see what was going on. When the rear heater is on, I notice that the rear dampener actuators are moving back and forth on there own. Letting cold air mix into the system. This is telling me it's a control issue. Next step is to find what controls those actuators, I'm guessing it on the console controls.