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.
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)
Hannes once: Green M3W; 2013 now: Red 4/4 Sport; 2011 and some practical cars for use in real life
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.