I literally just fitted an extra one to my Z4 last weekend. I took a feed from the wiring going to the cigarette lighter: should be safe as I have no intentions of using both sockets at once (I hated having the TomTom powercable trailing across the dash with the satnav in the middle of the screen). I routed the cable:
- from the back of the ashtray
- under the edge of the centre console towards the driver's seat
- down below the seat (I couldn't be bothered fiddling with the carpet, but the seat is flush with the centre console anyway and hides it)
- below the seat under the first aid box
- and I've just left it as a flying lead for the moment under the seat
(I intend extending this up and under to the panel containing the speaker (as suggested by cj10jeeper) but haven't gotten around to that yet...)
My satnav now sits in the bottom right corner of the screen with the cable tucked down the side between console and door, is tucked into the bonnet release handle, and runs along the floor to to plug in under the seat. No risk of movement as I've used some velcro to stick to the carpet, but it's all tucked away anyway.
HTH