Another thought, was the t-stat tested before installing it? I always boil some water in a pot with it in and with my Craftsman DVOM/thermocouple, verify it's opening temp and closing temp.

 

It turns out my bargain brand t'stat was stuck OPEN. Replaced with a good quality one. NOW have high temp readings on dash guage.

 

My AC is doing this - I dont think I have heat period. Gonna go in and check the heater core.. Thank for the heads up on vacuum lines. Completely forgot they were back there

 

Above the cab fan under the hood( passenger side) there is a white vac line that goes to the controls, now I'm not sure if this may help but it's worth checking out. My truck has the same issue and this is the only thing I've came across that looked troubling. Problem is I haven't driven my truck since I bought it so I can't be for sure. I know these 9gens have an infamous problem with this exact vac line tho.