After alot of though on how to get the line-out from my sound card into the cars stereo, I came up with a totally OTT solution, which was kinda fueled by ebay and several months away from the car and centered around an twin input EQ.
Anyway after more time than I care to think about, I finally finished installing the ICE which involved some 40 meters of cable (trust me I kept the runs as short as possible). Anyway I was testing it all as I was going along and every thing seemed to be going well. Then this afternoon I got to the point when I could sit back and listen to the fruits of my labor, turned it on and the head unit's line out died!
Plugged a walkman into the EQ and that all works fine (although I need the remote signal from the head unit to turn it all on). So I'm off to fix a head unit.