Can I plug my MP3 player into the aux. socket on my car radio, with the appropriate cable and play the stored music?

You should be able to do that as long as you have the right cable.

Most of the mp3 players have a "mini-stereo" jack (3.5mm) and some car stereos have the same type of jack for "aux" input, and others car stereos have RCA jacks for the "aux" input.

Even if the jacks are not the same type, you can use an adapter cable that converts the mini-stereo jack to an RCA jack. You can find them at many places like Best Buy, Circuit City or Radio Shack.

Depending on the type of car stereo you have, you would normally press the source button to change between the tuner, cd player, aux or whatever you may have connected to the stereo. You may even have a seperate button to press for the "aux" input.

I hope this was helpful to you, if you need more information just ask, and please give more info on the type of mp3 player and the make and model number of the car stereo.