This may not be exactly what you are hoping to here, but i have fitted a Tom Tom Go unit within the dash top storage bin.

The unit is hard wired in witha small hole under the rubber matting and the unit lays flat with the lid closed when not needed. It is a very tight fit, but the lid does close OK if the unit is laid flat,

I was a bit worried that the signal would not get through the storage bin cover when open, but it works very well and I usually have 100% strength. In and around the centre of London i do loose a bit of signal strength, but i guess this would happen anyway.

Tom Tom in UK at the moment are about £250, so this has worked out much cheeper than the BMW own unit, and suits my needs fine.