I wanted to be able to override the Power supply funtionality and be able to power off the PC even when the car is on so I mounted a switch inside the ash tray. I also installed a small USB hub to hook up thumb drives or mouse, keyboard. The hub is USB 1.1 but it is the smallest I found. It goes for $9.99 in Radioshack and is under clearance. The switch and the usb hub in the ashtray (specially in bimmers where the tray is so modular and easy to work with) has been done by so many others, but nevertheless it comes very handy.
The usb 1.1 hub in the ash tray is hooked up to another hub (USB1.1<<USB2.0<<PC) which I have hidden in the area under the shift cover. There are plenty of rooms there to run wires or install a device. My Xenarc touchscreen is also hooked up to this 2.2 USB hub.
I found these cable self-ahdesive brackets that came with an old computer desk to hold KB and mouse wires. I stock them to inner walls of the small carpartment area under the shift panel to hold the speaker and usb wires. see the bottom of the above picture.
