Easiest solution is probably a garmin etrex venture ($150-200?) which comes packaged with a pc interface cable (db9 serial) and a gender changer ($2-5?) to connect it to the sled's serial cable. It eats a pair of AA's in about 10-20 hours though. But then again, as it doesn't rely on the car for a power supply, you can use it for geocaching without needing a 4x4. Nothing will expect 12V on pin 4, that's an empeg specific feature.