Car phone E220

Hi there, I wonder if anyone might have any idea about a problem with a Nokia 6310 car phone ? I had to change the radio recently and the computer was reprogrammed to accept the new radio, the cd player and the car phone. The main options seem to be okay except for one issue - I can't hear on the speaker when a call is made or received. The handset is working OK when used out of the car and I can hear fine on the speaker in the phone. When the phone is in the car all the usual things work such as making a call and the radio mutes, it does all the dialling things etc. but I just can't hear on the car speakers or on the handset, if you take it out of the centre armrest and with the phone in the car cradle listen to the speaker as if using the phone as a normal handset. I wonder whether some-one might have any idea what may be happening here. I do make the assumption that when a call comes in, or I dial a call, I use the phone as a hands free system ? Any suggestions would be welcome before I go to Mercedes to have it looked at.
Regards, parkman