It also seems that using non-iPod MP3 players might be worth considering. There are a variety of transmitters available, although I note some have rather mixed reviews.Seems that the ability to roam accross a wider frequency range is important rather than a fixed 6 or 8 frequencies.
I would hope that having the ariel in the rear window, and placing any transmitter near the back of the seats (in the net) would get a good signal.