Whucky, try this site to understand the difference between Seatalk and NMEA:
Thomas Knauf ****SeaTalk Technical Reference
Seatalk carries power in the red wire. The data is the yellow wire. Ground is black.
NMEA is a on-way street, no back and forth communication. GPS would send its position data to the chart plotter. The autopilot sends and receives on the NMEA IN and OUT terminals. Seatalk is a Raymarine proprietary product, so the three wire cable won't do you any good when trying to interface with a non-Raymarine unit. You can try a Seatalk-to-NMEA multiplexer, but read the owner/installation manuals first to see if it will meet your needs. I've also wired all my NMEA data wires (the yellow wires ONLY) into a common buss (a common buss terminal, a brass strip with muliple screws) and output that to my Raymarine chartplotter via the NMEA input. By reading the previos reference to how NMEA works, you can see the logic of this. Even the Raymarine people agreed with me after I demonstrated that it worked.