If you have not already read these they may help. Not exactly your same situation but close. I was trying to get Smartcraft gauges and position data to a VHF radio. I ended up connecting both to the pigtail on the GPS antenna pigtail. All is working fine. The one thing that solved my problems is that one GPS output (talker) can be connected to up to 3 listeners.
One tip on NMEA 0183 is that there are 2 so called standards. One is differential and the easiest to get working the other is single ended. In my experience, as long as you are connecting diff to diff or single ended to single ended there is usually no problem. Connecting diff to single is usually a headache and may require a multiplexer.
Many manufacturers take the cheap route and use the single ended which almost negates being able to use 2 way communication. Humminbird's GPS out port is single ended. The Humminbird VHF is differential. Go figure, cheap on the expensive unit and expensive on the cheap unit, and a pain in the butt to get connected.
http://forums.sideimagingsoft.com/index.php?topic=711.0http://www.shipmodul.com/en/connections.htmlI attempted to get the GPS out working using the HHGPS cable also, after hours of frustration and head aches I settled for one way communication with the VHF using the GPS pigtail.
I read somewhere that you do not have to use the Y cable is all you want to use is the nmea. I tried it both ways and it made no difference. Here is a pin out for the HHGPS cable and the PC connect cable from Humminbird.