2001 rear air only blows cold air

I just bought a 01 suburban and just found out that the rear climate control only blows cold air. Doesn't matter what temperatue I set with the front controls. The rear air contrils were removed by a previous owner to install a DVD system so I can't check to see if the rear controls work. If I set the system to auto it just blows cold air all the time and on full speed. Manually the fan speed works fine. Just the temperature problem.

I also have noticed that the sensor in the headliner above the driver seat hums on occasionm but not very often. The front climate controls seem to work just fine.

Any suggestions?

Climate control is achieved by mixing heated air with the AC-cooled air. To me, it sounds like your rear heat is not working to mix in. Rear heat comes from the coolant, teeing off through hoses to the rear system's heater core. Maybe previous owner bypassed the rear heat because he had a leak.