Many car radios will play ipods via a cable connection. My Volvo has a small jack socket in the centre armrest and the car radio can be set to play the input from that. The radio handbook should give you the required info.
We have a Bluetooth thingy that plugs into the cigarette lighter and you tune it to a free fm frequency on your radio. It seems to work better with our iPhones than the iPods for some reason, not bad though and cheaper than a new car stereo.
I used one of the FM transmitters recommended by others many years ago. They are very good with the caveat that if you travel around the country the "empty" FM frequency that you tune it to can clash with a local radio station. If you can connect it with a cable that will be the best option.