Garmin sells these combination 12v cigarette adapter/serial cables on their website. They are pretty expensive. Some people are building their own and are often selling them on ebay.

But it all depends on what you want to do. Expensive gps's have builtin maps and as such can be really usable by themselves. But there are also things like the Earthmate, and gps-mouses, and things like the SV6 or Oncore receivers, that have no display and completely rely on a laptop, pda, or empeg , to handle the user interface, and in some cases need a custom built 12V->5V power supply or RS232 line level convertor. Tony got lucky there, nothing to solder but 3 wires.