I have ST1000+ tiller pilot with a ST60+ speed/depth/wind system and a Garmin 76Cx GPS. Everything works fine. I can have the ST1000+ steer to a way point from the GPS or steer to the wind from the ST60 wind instrument. Or just steer a compass course from the ST1000+'s compass. I have the autopilot's status messages popup on the depth display.
Connect the Garmin's NMEA out to the autopilot's NMEA in (+). Connect the autopilot's NMEA in (-) to ground. The autopilot does not have a NMEA out, but there are two wires for the in because it is true NMEA 0183 and uses differential signalling. Connect nothing to the 76Cx's NMEA in (which is basically useless in the 76Cx). Connect the autopilot's seatalk to the yellow seatalk wire from your instruments. Connect the 76Cx's ground and 12V wires even if you plan to use the batteries in the 76Cx.
Make sure the ground is the same to everything. This means don't run anything that draws a lot of power on the same ground wire as some but not all of the electronics using too little gauge wire. The autopilot can use a fair bit of power, so make sure you used the proper gauge wire for it, or it won't have the same ground as the GPS or instruments.