I have a 2000 S10 that blow cool air - replaced thermostat - did not fix the problem- how do I change the "heater valve"?

I have a 2000 S10 that blow cool air - replaced thermostat - did not fix the problem- how do I change the "heater valve"?
Is the vehicle reaching operating temperature, temp gauge reading correctly?
Does the mode switch from Heat(floor) to Vent and to Defrost?
There is no valve, but the doors inside the dash move to blend cooled and/or heated air. Check for movement by switching from Heat (Floor) to Defog/Defrost. If the mode isn't switching, check for vacuum leaks (single black hose that passes from the engine, along the harness into the passenger compartment).

http://aycu21.webshots.com/image/30820/2003695166761713342_rs.jpg (http://allyoucanupload.webshots.com/v/2003695166761713342)