THere are two things that control the heat.
First is usually a coolant valve that shuts off the water flow to the heater. Usually near the firewall where the water lines go through.
The other is a baffle that diverts air between the A/C and heat sides of the plenum. If you set the temp on the dash and it adjusts automatically, then this is electronic. If you just have a cold/hot knob, that usually hoes straight to the baffle.
Edit: looks to be electronic and no valve:
Embedded Systems Consultant,
SHDesigns home - DIY Welder