If you have an old adapter then easiest thing to do is take apart the cigerette lighter end and see where they terminate.

The wire that terminates on the tip (centre) is the live and as Chris said needs a fuse.

The one that terminates on the outside is the earth.

Bare in mind some of the newer systems actually charge the unit.. IE TOM TOM and they have 3 wires and you cant do what you want to do.

Jake