The question comes up so often about running 12 volt devices on a 6 volt positive ground car. If you do ebay here is a very good little device that I use often. They work very well and are very durable. The pc board is epoxied so vibration does not bother. Will put out a solid clean 10 amps, available in 20 amp versions as well. For anyone looking at adding a radio, two way radio, cigarette lighter power jack, etc theses are a good quality, low priced option. You will want to have an on off switch on the input otherwise will eventually drain your battery. 95% efficient too so very little heat produced.
http://www.ebay.com/itm/Voltage-Boos...wAAOSwxp9W7~1F these are also available in 24 to 12 volt stepdown, 12 volt to 24 volt stepup, and others.