you are right - the car will now ignore the MID as the nav is fitted. You must ask the dealer to disactivate the nav, and code the car for a MID.

OR

Make your MID a FMID - the REAR MID, as fitted to the rear seat centre armrest on the 7 series.

To make the MID think it is a rear MID, just connect pins 5 and 11 together on the MID plug. It will then work with your whole nav system as a remote control :-)

PS: then you could keep your MID and install it in the rear seat area of your car.
All you need to connect the MID is permanent power, ignition power,illumination, ibus and ground