I do this all the time with a Calrad pre-fab adaptor that is anywhere from 6" to 18" long, depending on the stock they have had manufactured. I usually install a five conductor cable with BNCs, using R and G and B for component, and using B and W to carry S video. The adaptors come either with BNC male, BNC female, or RCA connectors. I can get these for you if you cannot find a vendor who carries them. I believe Calrad is carried by A LOT of electronics parts vendors nationwide, and they should be able to order them as well.

Incidentally, I personally discourage using RG-6 for video unless you have the tools and connectors to crimp on BNCs (Calrad has this, too), because you can't solder to its aluminum braid or shield. If you have to terminate RG-6 with an F connector, your connection will sooner or later be flaky with video. RG-59 works well for all current video frequencies. I used to use it for all VHF antenna usage, and that goes up above 200 mHz, so it should be okay for video.