Original equipment power sockets are often cigar lighters wired from the fuse box on the same circuit as the clock( if fitted), usually at the right of the dashboard for RHD cars and at the left for LHD cars.
New or additional sockets can be placed wherever convenient – the inside of the glovebox is a popular choice.
Owners may decide that the cigar lighter socket is not robust enough, especially if they have a battery conditioner or tyre pump, and to fit a separate socket.
Behind and between the seats (in a two-seater) is a convenient location, with the socket fixed near the battery cut-off switch.. Plugs and sockets are inexpensive and readily available, for example from Maplins or Radio Shack.
If you fit a ‘non-standard’ socket, remember that you will need matching plugs for all the items you will be using in it, or an adaptor to permit the use of items fitted with cigar-lighter plugs. The wires to the battery must be of suitable quality and the live wire must be fitted with an in-line fuse.