When I was in the States over Christmas / New Year, I bought a Sony device, which connects all sorts of MP3 devices to it, either via a mini headphone jack, or mini USB socket for charging. My laptop, phone, and MP3 player are all connectable.
There are 4 channels on it, which can be pre-set to any FM channel you choose. The antenna connects magnetically to the seat rail, thus turning the car into a mobile transmitter. The signal is then picked up again by the car radio antenna, and played as a normal radio station. This can be most interesting in areas between transmitters, or anywhere that a channel becomes weak.
If you find yourself at a standstill in traffic (the A1 stretch around Blythe roundabout is particularly good/bad), you may well find Alex Jones coming through the airwaves instead of hearing the traffic report on Radio 2 - if so, look out for a blonde driving an Alfa Romeo
Aah, Friday afternoon traffic jams on the A1 are a lot more fun than they used to be
Edit: Here's a
link to the Sony gizmo I use. It really does work well, and has better signal strength than similar transmitters that a couple of friends have tried. I'd still be interested in boosting it though!