It just doesn't seem very clean to do--I mean, you have to CUT wires. To me, keeping these wires as stock as possible is the best way to go. Of course this route is always possible but, there are many advantages to just getting a new HU versus the disadvantages of the converter box.

 

Is a "line out convertor" what he's after? If so there's a new one on iWsti.com for cheap:

http://www.iwsti.com/forums/showthread.php?t=45134