Daewoo Nubira Heater stuck

2000 Daewoo Nubira, 60k miles, A few months back the heater would take forever to warm up and would never really get hot. Now it is very hot and wont get cool when dash control is selected for cold.

Does anyone know if this particular car has a heater control valve? and where I might find it? What else should I be looking at?

Thanks