You can keep your steering wheel buttons with an aftermarket deck... I did.
What you do, is get a device, very similar to one of those remote controls that "learn" from other remotes. I forget the brand name now, but I've got it installed in mine.
Basically, you connect the wires on the back of the steering wheel to this device which has an IR port on it. (the aftermarket deck you get must come with an IR remote btw).
You follow the instructions and "program" the box. Basically, push one button the the remote (eg volume up), and the same button on the steering wheel. The box will have a rather long cord, with an IR device at the end of it. Mount it somewhere close to the IR port on the cd/dash head unit. Now your steering wheel controls mimic the remote.
This allows you to keep your steering wheel controls and even improve them. My 91 came with a CD player, but the steering wheel controls didn't have a Track Fwd/Track Rev... it only have <- SEEK -> which only FF the song. :( But when I reprogramed it, I programed the <-SEEK-> for track FWD/REV, and I even reprogramed FADE UP/ FADE DOWN, to be "Directory" UP/DOWN (since I have an MP3 deck).
If you need pix, I can grab the camera and take a few for you. I'll try to find out what the name of the device is, and/or brandname/make/model. I'm assuming any car audio shop would know what I'm talking about.