I recently replaced the thermostat on my 2003 Jetta 4 cylinder. The engine temperature is normal at 190F. However, I cannot adjust the inside blower temperature. It is always hot. The fan works and the heat can be directed to defrost, vent and floor.
The heater control in my 2003 VW Jetta only blows hot air. Cannot adjust cooler on 2003 Volkswagen Jetta
The temp control is toward the top of the heater box. It can be accessed behind the stereo and heater control head. These parts are plastic with a cable control. You might remove the stereo and control head, and while turning heat control, see if it is moving properly.