I was thinking about hardwiring the aux in connectors on my sled to a phono jack mounted in my car, but I can't seem to find an appropriate schematic anywhere. I realize that the RatShack could probably sell me a cable that will do that without me needing to deal with it, but I'd rather get a mountable jack, and that won't be possible with their cable (of course, it now occurs to me that I could get such a cable and cut it open, but I've already typed this much now).

Can anyone point me to such a schematic? It should be quite simple, I imagine. I just don't know how to go from the separate to a shared ground.