I have measured the voltage on the pins by connecting a multimeter on the +ve battery terminal, and then on the pins.
One pin came out at 12.4 v, the other at 7.4 v
I phoned Garmin UK, who said it should by over 12v on both pins, so they are sending out a new wiring harness.
Hope that cures it. Give them a ring and they should help you out