Please explain what your trying to do again, what you said didnt really make any sence but from what i got out of it your trying to power some headlight globes outside of the car for something else?
You need to work out how many amps your circuit will draw so you can choose a plug back adapter.
Amps = Watts ÷ Volts.
So using that formula if you have a standard headlight globe which is 55 watts you would have a current draw of 4.5 amps for each globe. Remember tho that a cars electrical systems voltage can vary abit. Normally when the car is running it will be at about 14.x volts, as the voltage rises the current draw will lower so you dont really have to worry about it.
Plug back adapters that can supply a decent amount of current 10 - 15 amps will be rather expensive IF you can even find one. I would recomend buying a AC - DC power supply , something like this will do for most applications.
http://www.jaycar.com.au/productView.asp?I...mp;SUBCATID=381
or
http://www.jaycar.com.au/productView.asp?I...mp;SUBCATID=381