Most manufacturers use a stand plug configuration these days (ISO10487) which looks like this
http://www.mictronics...ker/becker_pinout.jpg
So you can either get another head unit that conforms to the ISO standard, or (as twr says) just buy an adapter lead to convert the stereo plugs to the ISO standard on the cars harness.
that said.... the head unit in most modern cars are actually pretty good, if you're doing it for better sound I'd look at upgrading the speakers first.