The control for the climate control upper,lower,Defrost on my 2000 Yukon does not work.The air just blows out the upper vents ,no defrost or foot heating or air. Can someone help with this problem. I have replaced the cabin filters. There is a unit on the passanger side footwell near the hump.Could this unit have gone bad?Thanks for any help.

Replace heater relay or just swap the relays and then see if it works
Take heater relay out and put a/c relay in its place if this works you've
found the problem if not its the heater core control look it up at autoparts
Web site so you know what your looking for also get a
Book post your results A REPAIR BOOK FOR YOUR VEHICLE