Maybe this will help...
Although my set up is a bit different, perhaps my experience will help a bit. The wiring is likely the similar if not identical. My objective was to interface a Garmin GPSMAP 76CS with a Raymarine Tillerpilot ST1000 Plus. Specifically, I wanted to power the GPS from the boat's 12 v. power supply, as well as drive the Raymarine tillerpilot through the Raymarine 6-pin socket. For future reference, here's the correct installation, which I gleaned from the responses above, and from the technical folks at Garmin (who put me on hold for 20 minutes.) Note: the interface information as written on page 50 of the Raymarine Tillerpilot Owner's Handbook will not work for this particular application.All wires refer to the Garmin Power/Data serial port cable with bare wires (round data port end inserts into the GPS):Red wire 12 v. to socket pin #1,Black wire ground to socket pin #2 AND spliced to socket pin #5 [NMEA (-)] This is the real trick of the installation.Brown wire (NMEA out) to pin #6 [NMEA (+)].White wire is irrelevant for this application (can be left unconnected), but can also be lead to pin #5, which I did.On the GPS interface page, the GPS is set to NMEA In/NMEA Out, 4800 baud. Find a waypoint on the GPS, hit GO TO, press Auto on the tillerpilot, the press -10 & +10 simultaneously, wait for the response, then press -10 & +10 again to activate the tillerpilot tracking. Ta-Da!! That's how it works on my set-up. As they say, your mileage may vary.