Hi, my plotter was only a 451S but this was my solution copied from my post in June to the same problem. Should be close enough to give you a start? The instructions with the iCom 323 seem to over complicate it and I remember how frustrating it was :-)
Tested iCom's instructions yesterday and yes, contrary to the manual which talks about channels A and B etc, it is actually as simple as the following:
For GPS data to the radio:
Icom Yellow to Garmin Blue
Icom Green to Garmin Black / Common Ground
For NMEA data from the radio back to the Garmin:
Garmin Brown to Icom White
Garmin Black / Common Ground to Icom Brown
All working
Thanks for the input from everybody.