Well, anything involved with heater core and hot/cold air flow is usually mounted right next to it. The flap or actuator that meters air through or around the heater core is most likely the source. No 'cooling water' is utilized in bringing air temps down, air either goes through or bypasses the heater core. Yours seems to be stuck on flowing through it. I'd inspect all the parts to make sure nothing is broke first, if it is fairly easy access.