The fag lighter on your car needs a high current, so splicing the wires into any 12v supply is NOT a good idea. If you want to fit one, use a cable thats capable of handling 30 amps and go straight to the battery positive terminal - but you MUST put a fuse as near to the battery as you can to protect things. The earth should also be a thick cable, going to a suitable point on the bodywork or direct to the battery negative terminal.
John
January 2008