If the Autohelm is a Seatalk model i.e. ST3000, you will need an Interface Box. This can probably be obtained from ??whoever (Someone supply the name please) has taken over Autohelm.
If the 3000 connections are similar to my 2000, I can email you the connection diagram. If it has a three pin plug/socket, two pins are + and -, and the third is data. This goes to the interface box, which is between Autohelm and GPS.