connecting a e80 to a c80

guys,
I have a c80 in my boat right now with a fish finder and gps anteanna. I just received a e80 and radar. I want to install the e80 and radar and then link the 2 together to share the gps anteanna only. I know data will share but not video.

Do I connect the wire from the seatalk rgy on the back of the c80 to what on the back of the e80?? The seatalk/alarm out or to the nmea0183?

I know I will have to splice the cables I think....

Please help....