Having fitted a windscreen camera and also going on long journeys requiring my iPhone to act as a SatNav, and needing to be plugged in to charge, I found one auxiliary socket was not enough and there were lots of untidy wires hanging all round the passenger compartment.
Also my existing auxiliary socket was wired directly to the battery and for both the camera and the iPhone I really need them powered up only when the ignition was live. Time to sort it out.
So first thing to do was buy one of these from E-Bay.
Then I mounted it on the electrical panel on the bulkhead in the passenger footwell to add to the 'rats nest' of wires already in there.
(Note Battery voltage reading 12.2V so still good)
Here's a close up with the USB Charging lead in the left socket and the Camera power supply in the right socket.
And the USB lead just popping out from under the dash.
Resulting in a nice clean finish with minimal wiring showing, both the camera and the iPhone charger power up only when the ignition is turned on and I still have an auxiliary socket for other uses (such as blowing up my mattress when I take her camping).