Suburban is garaged in CA, central valley. I am original owner 173,000 miles. The car has been well cared for. HVAC works as it should until I take it up to Lake Tahoe in winter then it acts up. It quits blowing air out of anywhere. No defrost, vent, floor. I can hear the blower blowing. Fan speed responds to switch. I've even driven up and left it on defrost, parked it, come out in morning and no luck- no air. I can see the actuator on the drivers side moving according to my inputs. I've tried every mode, recirc, A/C, heat level, nothing makes a difference until it comes home as sits over night.I see nothing dripping or any other symptom. This makes for a foggy windshield and cold drive at Tahoe in winter. I've looked down the vents but don't see any issues. I see what looks like a relay mounted on top of HVAC hosing, passenger side, what is that for? Any ideas?