I think I've got a bad Temp Control Valve causing my truck not to have heat. Where is this piece, and what does it look like??

 

Wcmo, black plastic junky looking think under the hood, right rear of the engine compartment. Will be inline isolating one side of the heater core circuit. Will have a small metal lever connected via a linkage to a vaccum diaphram which in turn will trail a small diameter pigtail of hard vaccum line.

Hope it helps.