To pipe the stock headunit through the empegs AUX in you'd need a high level to low level converter (speaker level to line level) which takes your speaker outs and gives you RCA outs instead.

To use the steering wheel controls... For a MkI I think you need a custom bit of circuitry in the for of a small microprocessor that translates keypresses to serial commands which you can pipe to the serial connector. This has been done, but I don't recall by who - search the board... Or, possibly some sort of interface with an IR-transmitter that sends IR-commands...

/Michael