The problem is either the heater control valve(least likely) or the climate control pushbutton unit(most likely.)

By applying voltage to the heater control valve, you can tell if it is shutting off or not. It is a normally open valve, so if it doesn't get a signal from the CCU it will flow hot coolant, thus hot air from the a/c. If it is shutting off okay, the problem is most likely the CCU or a bad connection between the two.

DO NOT replace the CCU without first measuring the current draw of the auxilliary water pump. Disconnect the aux pump and connect voltage to it in series with an ammeter. If the aux pump is drawing more than 1.3 Amps replace it when you replace the CCU. Failure to do so will result in another failed CCU.

Hope this helps,