In my rv-7 I installed a AUX port (cigar lighter set up ) 7 bucks at radio shack power comes from the master buss thru a 10 amp fuse it works great i have had serval items pluged into it
If you are limited in space for the socket and/or do not want the regular cigarette type lighter socket, you can also get some small DC plugs and jacks at "The Aviation Department" of Radio Shack. {
We have that in our RV6 and copied it from a friend who has it in his. You then make a pigtail that has the big cigarette type plug on the other end and use that when needed.
