I have a stereo transmitter, bought on Ebay for £7.00 which plugs into the cigarete lighter socket and transmits on a fixed frequency of 107.7 FM.   I think you can also get multi frequency versions to overcome regional problems with different stations. It plugs into the headphone socket of the IPod (or CD walkman if you still have one of those old fashioned devices)and the quality is excellent, unless as already mentioned the IPod battery level is low. On long journeys to and from festivals with three children in the back of the car I connect the transmitter to the laptop so they can watch DVDs complete with four speaker surround sound..!

Chris