Yes I did this, I have an old Seat Ibiza with a radio cassette head unit. This had a CD changer wiring in the car, but no CD changer. I used the CD changer wiring, shorted out the changer wires to get the radio to recognise AUX, then soldered in left, right and ground to a 3.5mm input jack and I can now plug in my N800. Better sound than an FM transmitter.

There are also a system called connect2, that works with most radio's, search google for it. I've not used one, but I had a look before taking a soldering iron to my car radio.

Rich

Are these available in the US ? I've had a hard time locating them here...

Try mpd. It runs as a daemon to play music. You control it with a separate frontend, such as mmpc, or the command-line mpc. (Don't panic, you don't have to fire up an xterm to pause mpd; mpc is typically used in conjunction with some other program -- a frontend for a frontend, as it were. I use powerlaunch's mpc mode.) The great thing about mpd is you can start and stop the frontend, or even run two frontends at a time without affecting mpd. The state only changes when you issue a command from a frontend.
It will, I think, lose it's place if you reboot, but I'm always on with a charger at home and one in the office. I only shutdown overnight, so no problem for me.
My setup is mmpc for controlling the playback when I have the N800 out, and powerlaunch's mpc mode when it's in my pocket. I'm not sure which I like better for in the car.

http://europe.nokia.com/A4460119

Aww, damnit, I wish I would have seen that before now. I've got the official car charger, but it never occured to me that they might make a FM transmitter, especially one with a charger built in. That's pretty nice.

Anyways, I use a FM transmitter daily, and it works very well. Look for one that can transmit on any frequency. Some only transmit below the 92.* frequency, which can be bad, since the dial is typically pretty crowded with low-power stations in the lower frequencies, at least in my area.