Hello, I have a 2000 Yukon XL 2500..front and rear climate controls... front has no heat, rear works fine..all fuses for heater and front hvac are good...if you turn up rear to high and let it sit with the front control on hot and fan off, you can feel heat coming out of front vents until you turn the front fan on,then it blows warm air for about 10 seconds then goes cold again...maybe a celenoid for damper door? thanks for the help...

Hey Dan, this is not for your yukon, BUT - its a great diagnosis guide for GM A/C systems...
1995 Chevy Van G20 Heat Blower Fan Repair ZEDIC COM

Is your heater blower motor working on all speeds ? Are you get any air coming from the rear vents ?

There's a rear hvac fuse 30amp that powers an auxillary blower motor processor controller. The controller has the resistor to regulate fan speed. The front and rear fan control both pass thru the front controller first then to the rear processor.
You may need a shop manual to be able to diagnose the problem.

Could be a bad ignition switch.. needs to be checked by a professional.

Rear heat not working ? The blower motor isn't working ? No air coming from the rear vent's ? Is it just a yukon or XL or denali ? What size engine ? 4.8 , 5.3 or 6.0 l Manual or automatic climate control ? Does the blower motor work on any speed ? The front an rear heater system are seperate , have there own electrical circuits . Could have a couple of different systems ,could have a control processor or relay's an a resistor block . Do you know how to test automotive electrical circuit's using a wiring diagram and a DVOM - digital volt ohm meter ? Plus to get at the rear heater-A/C unit you would need to take the right rear trim panel off .
2003 Tahoe No Rear Heat Fix Suburban Yukon XL Repair

Could be the HVAC control processor is malfunctioning.Wtihout testing that would just be a guess .
2003 Tahoe No Rear Heat Fix Suburban Yukon XL Repair

YOUR REAR AC AND HEATER CONTROLS SHOULD HAVE RR HVAC FUSES IN THE FUSES RELAYS BOX UNDER THE HOOD IN THE ENGINE COMPARTMENT.

{: ) If the front a/c and heater is working fine, that's good. At least the problem can be isolated on the rear HVAC circuits only.
Check first the RR HVAC fuse 30amp at the underhood fuseblock and the IGN 3 fuse 10amp at the IP fuseblock. If you found them good, then you can begin diagnosing the blower circuits wirings and connectors.
A faulty blower definitely will not give any output. If it doesn't spin for some reason, rear a/c or heating cannot be circulated.
A faulty blower resistor or speed control processor can also cause a zero air output. Sometimes, the blower may only work at certain speeds only.
A faulty rear HVAC control switch may not give the proper signals to the blower resistor or speed controller, and in turn will render the blower motor disabled.
Sometimes it can be a terminal connector being loose or burnt. However, the most usual cause for this type of problem is a faulty blower resistor.

Does the front blow cold ? Diagnosing rear problems
Taking to a qualified repair shop an having it diagnosed would be your best bet >

check and make sure the duct work going to the rear vents are attached and in good condition