Here's some pictures of how I wired a TomTom iPhone mount behind the dash of a VW Passat.
Note, this didn't involve finding a new power source or wiring to the fuse box. Instead power was taken from the 12v lighter/accessory socket in the ashtray. The actual ashtray was removed (it is designed to be removable) so it can be put back at a later date.
The jack end of the tomtom power cable can be stowed in the ashtray cavity when not in use, with the lid closed - hiding it from view.
To perform this job requires a couple of tools designed for the job. Ideally you need "dash tools" - these are strong plastic wedge shaped tools that allow you to pop the fascia off the front of the dash. The fascia in all modern cars is just clip on plastic. You also need a suitable star-shaped spanner set - as most car fittings use this form factor. This is used for removing bolted in items such as the ashtray container and air conditioning controls. A set for about £20 is a good investment if you intend working on your car a few times or on several cars. Finally, i also used a "magnamole" - a new invention (as shown on Dragon's Den") of bendy flexible sticks with a magnetic end - very handy for routing and picking up wires in small spaces you can't get your hand.
The tomtom itself is mounted on the dash using the standard suction mount onto a tomtom-supplied sticky disc, designed for the purpose. They cost about £5 for two. (I've also used one to mount a video camera in the rear of my car)
Pictures of the installation follow:
This was a bit tricky getting the USB end through the small hole. Ideally I'd have gone through the gap at the side, but it was just too small.
This was the bit that needed the Mangamole to pick up the USB cable from behind the lighter power socket. I removed the A/C controls and dropped the rod down behind to grab the metal end of the cable from inside the lower part of the dash.
Drilling the hole was unavoidable - the ashtray cavity is totally sealed and any attempts to come round/over the side/top prevent the flap from working. However, the actual ashtray has been removed (it's designed to for cleaning/emptying) so the hole is easily covered if the cable is removed and the ashtray replaced.
The final job is pretty neat and discreet. It avoids those tell-tale suction marks on the windscreen that thieves love and with or without the mount in place is easily covered with a small hat :-) It is also much safer for driving as there is no risk of the trailing cable getting caught up in anything (gear lever, hand) and the iPhone is in much closer reach of the driver.
The power cable stows neatly in the ashtray when not in use and is plugged into the lighter socket when required.
The positioning of the iPhone tomtom also improves handsfree performance as both speaker and mic are closer to the driver.