You can buy a steering wheel control module for any aftermarket brand stereo from a couple different companies. I've used PAC modules in the past. The universal SWI-RC will control virtually any brand aftermarket radio with a remote input.
If you want to buy used and save a few bucks, you can still find the old school application specific modules too. The SWI-JACK is for JVC, Alpine, Clarion, Kenwood, and Blaupunkt. The SWI-PS is for Pioneer and Sony. Finally, the SWI-ECL2 is for Eclipse.
They also make a non brand specific SWI-X model that can learn infrared commands from the original remote and control anything that has IR capability, so you can even control really oddball stuff. I set up a car computer one time and programmed a SWI-X to emulate the Windows Media Center remote.