i have an FM transmitter for listening to my MP3 player in my car, but is it possible to have a direct socket [such as is on new cars] installed in the car?
Yes it is. But you need a new stereo. I did it in my car and haven't looked back. The older stereos don't allow for an extra part to be fitted. My advice is go for one of the new model pioneer stereos, they seem to be the best option. And even the cheapest ones have the external fitting for mp3 players.