Have you got a new phone recently? I would consider trying a different phone if possible, they are not supposed to cause harmful localized interference. Does a friend's phones of a different model cause any problems? I'll bet the problem did not exist when the car was new - there have been so many new digital signaling schemes (TDMA, CDMA, EVDO, etc.) developed over the past several years the digital signaling causing the problem today probably did not exist. Working for Motorola, I'm a bit partial toward their phones
PS: Sorry for the big signiture picture, I have not figured out how to scale it down yet!