Hi.
I fitted a Highway 600 a couple of weeks ago.
I'm pretty new to fiddling with cars in any meaningful sense.....
But managed to hide the cables behind the windscreen pillar easily enough (without breaking the infamous plastic tab at the bottom of the pillar trim). Carried on the cable run down behind the weather seal .... then behind the side foot-well trim and then 'up and over' .... behind the plastic trim above the foot-well but underneath the glovebox itself. You can JUST squeeze the cable through a very small gap over the transmission tunnel to the driver side. You need to drop the trim panel under the dash .... very fiddly screws to remove/install. From there its easy to tuck the cable along the trim panel that leads to the 12v cigarette lighter socket.
It all looks pretty neat. I used gaffer tape to secure the loose lengths of cable. The Z4 has enough rattles already, I reckon!
Removed the trim along the roof to separately route the microphone to the driver sunscreen.
Really pleased with the unit. I installed the wireless control box at the corner of the of the center console ali panel in the corner just above '1st gear'.
However, the sound quality through the AUX input is horrible. Very distorted. I've changed the AUX input level on the BMW Business head unit to its minimum setting but it's still distorted. Perfect digital radio audio over the FM transmission channel though. I'm happy enough with that way of doing things.