Over near the battery is the mono-water valve. Kind of square shaped if looking from the top, and a silver colored "button" on top, with 1 two-wire connector. Try removing that connector and see if you get full heat (uncontrollable by the heater controls at all), OR if it acts the same way. If you get uncontrollable max heat, you have a fairly involved problem, probably relating to the radio work.
If it acts the same way, there is a problem with the mono-water valve, but it probably just needs the repair kit.
Let me know the results of disconnecting the wire from the valve and I can tell you how to take apart and visually inspect the water valve interior, or use the search function, it's been discussed many time before.

Gilly