So I took the car back to the radio shop and got nowhere. They said they couldn't fix it ,and it was unlikely to be caused by the radio installation. I know better, as it had always worked perfectly right up to the minute I left their shop. What it comes down to is that once the temp achieves the 22 degree setting the heat shuts off and blows only cold air and heat will not return until the ignition switch is shut off.
I found that if I slightly turn the switch towards the off position while the car is running it will reactivate the heat. The installer said he phoned the Mercedes dealer and the dealer said that installing a new radio could not effect the operation of the climate control. I don't beleive that story, as no shop in their right mind would defend this situation.
Kind of looks like I'm flogging a dead horse here, and will most likely have to try to resolve the problem myself.