Rear climate control not heating 04

my 04 burb`s rear climate control is blowing a/c even when the temp control is set to heat. Front driver and pass are working fine. Just can`t get any heat out of the back controls. Any help would be appreciated by my 2 month and three year old. I am having to shut off the rear system and crank the front to warm them up.

Same thing happened on our 05, dealer had to plug in there machine and reset something?

Jeff