Interior Air Temp Sensor

[garwar]

My 2003 GMC 1500 HD has the digital automatic heater/ air controls. Well the heat only works when then temp is set on 90, any thing below and you have air conditioning. The dealership told me it was interior air temp sensor. I bought a sensor, I just now need to know where it is located. My best guess is behind the filter under the dash but I figured I would ask before tearing into the dash

 

 

Thanks,

Gary