Join Date: Apr 2015
Location: North Central WV
Mentioned: 0 Post(s)
There's nothing particularly complicated about a 12V outlet, cheap or expensive. Any waterproof 12V cigarette adapter from anywhere, even WalMart, will be fine for you. Lots of people use ones from WestMarine.
Maybe you're trying to plug directly into a harness versus splicing into an existing 12v line, if that's your question I can't help you much but that's not how I'd do it anyway---I'd save the harness for a different application.
Relay is only necessary if you're splicing into a line that can't provide the power you're looking for---but you're looking for a phone or camera, not a refrigerator, so you should be OK.