A push in connector used in vehicles on cigarette lighters. Commonly used as a means of powering other equipment in a vehicle, such as mobile phone chargers, CD players etc. Note that the same connector is used on 12V systems in cars and 24V systems in lorries - this requires care not to damage the equipment!
Cars, Vans, Caravans etc. (12V)
The centre connection is positive, the sprung 'wings' on the sides which hold the plug into the socket are negative.
The connections are normally solder terminals (the sprung negative connection may not actually have a terminal, just wrap the wire around it in the centre and solder it). Remember to thread the cover onto the cable before soldering.