Autoleads in the UK do one. Cambridge Car Audio fitted my Empeg so they would know. My standard head unit acts as a Source and is plugged through the Aux on the Empeg, which I think is what you would like to do.

www.cambscaraudio.co.uk and email them from there. They are very helpful. They have an internet sales outlet www.incarexpress.co.uk and are active on forum uk.rec.audio.car.