rear heater blows cold only on 2003 GMC Yukon

I have checked the hoses in and out from the rear unit, ian all of it's functions work, but it will not shift from cool to heat. Any suggestions?? Chris

Asked by for the 2003 GMC Yukon
My 2003 Yukon did this. Pulled the passenger rear access panel and blend door motor wasn"t working. Tightened the black ground wire by the blower motor and now everything working fine.
1 more answer
I would get the diagnostic trouble codes checked to see if there is problem with the control system. A check will also need to be performed to see that the engine thermostat is operating properly and that hot engine coolant is getting to the heater core in the rear unit.
Let us know what you find out!