Originally Posted by crystalworks

Yessir. The harness that originally connected to the head unit in the battery/electronics tray needs to run all the way up front to the new Seicane unit. They should have provided, or sold you separately, an extension harness for this purpose.

The reason for this (as I understand it) is that the Nav unit in the E53 is really just a display unit (and tape deck) for the actual radio in the trunk.