Not sure this is worth the effort, but.... You need to find a cigarette lighter adapter that puts out 5-5.5 volts (not tough, most PPC ones are in this range) and find a female PS2 socket. The pinouts are easily found on the web (
http://www.bbdsoft.com/keyboard.html ). Then you can power your GPS. Of course you haven't powered your PPC with this arrangement. You could hack through a serial sync/charge cable to splice that in (Pinouts likely available at Beverly Howard's site). At that point you have a very nice kludge, if you haven't messed up and fried something.
You could also go to Deluo and get a GPS designed for your particular PPC.