02 Envoy rear heat problems

Hi

The rear heater in my Envoy works when it wants to, for how long it wants to. This has happened since I bought it 4 yrs ago. It comes on once in a while while driving, usually lasting about a minute before it shuts off. Playing with the controls (both front and rear controls) does nothing. This morning it came on again (first time in 5 months). After shutting off the vehicle and opening the door, the heater did not shut off. About 2 minutes later it did.

Anyone know what to replace to gain control of the rear heater fan?

Thanks :)