In theory, it's not too difficult at all. Find the wires coming out from your steering wheel buttons, send them to the analog inputs in a small processor (PIC, perhaps), and have the processor convert these into RS-232 signals to be sent to the Empeg. The Empeg allows single-character controls from the serial port while it's running.
In practice, you need to be pretty good at hardware design. I'd recommend 2 things if you're not:
1) Look on the Projects forum of this BBS for "Big John's" stalk remote project. He also has details on his web page.
2) Buy a Kenwood steering-wheel remote for $30US.
HTH

_~= Dearing =~_
"WAY too happy about having #99."