Usually two wires. 12v +ve and earth, but of the right spec to take the load of whetever you plug into it. Some have third feed for an internal light. They can be taken from a fuse, or even the radio feed (subject to load comment above).
If wired permanent live, can also be used to plug in battery conditioner. Arguably safer to be switched live. If permanent live it's wise to fit a cover to avoid something getting inside & shorting it out.
