The plot thickens, because on the Garmin website there are two versions of the instructions. If you look at the diagrams on pages 5 / 6, the thing that has changed is, (guess what!) in the newer version the + supply is no longer connected to the GMI 10 via the NMEA 0183 connector - only to the GPS sensor.
The two manuals reflect a change to later versions of the GMI 10, which get their power via NMEA 2000. It seems that if the OP has the newer version, he is using the old, wrong, instructions.