We needed a 3A USB socket for charging mobile phones, GPS and iPad in the car. Wanted it to be hidden while easily accessible.
If it would be of interest. This is the way I arranged it.
1. Bought a 12V to 5V (DC-DC) (3A) converter on eBay.
2. Connected the cables behind the 12V outlet in the glove compartment. Black (ground) to black and red (+) to red (light) on the 12V outlet. I want to be able to charge when I have the lights in the car on.
3. Glued converter and plug firmly under the glove compartment.
See pictures.

Nice idea.

Good idea, but why not plug a USB adaptor into the 12v socket? At least, mine has one,under the dash below the glovebox, maybe yours doesn't?

I have got a double outlet USB adapter. Fits perfect with the 12V socket.
(But I did add a double outlet USB socket parallel to the 12V plug to my M3W)

Having done this for two reasons.
1. Adapter for regular withdrawals are normally only at 2.1 A
2. I wish only to charge when the light and the ignition is on.