AC/control switch

2000 GMC only comes out on feet can not get any air out of top vents
I can hear the blower moter kick in when i put it on MAX but still only at my feet. same with heater.. can you tell me where the control is located to switch it seems like a vent flap is not opening to the top vents


and the wife is on my case to fix it up...

I recently had a similar problem, but the air would only come out of the defroster vents. It's cause was a broken vacuum line that I broke while replacing the thermostat. They use this crappy hard plastic line (it's small, looks like electrical wire) that dries out and cracks when you touch it. Find the broken vacuum line and splice it back together with some 7/64" windshield washer pump tubing.