Thanks for your reply.

I dont know if the the system has an aux in or not. Actually it does not matter much, because as far as I know there is not enough room for another unit in the dash, plus I'd rather go the opposite way of running the existing headunit through the empeg.

As I understood it, it is somewhat complicated. The navi-system uses a CD stored in a 5 CD changer. The changer itself is not located in the trunk, but in the dash, too, taking up a DIN-slot. So with a little magic of an experienced installer it might be possible to relocate the CD change to another place.
I could then install the empeg in this spot, but then there might be not enough room to pull the empeg out.

I hope the above makes some sense!? I sometime have problems expressing my thoughts in english, so it ends up more as a stream of consciousness.