Both the manuals on the garmin website state that the 5 pin connector is for future use.
I reckon - (and this is just from reading the manuals i downloaded online) - that your best bet is to try the grey lead from the main 18pin connector with the black as the ground.
This output seems to be able to feed a gps repeater or autopilot - (im assuming you are already not usaing it for an autopilot) - so it should simply be an NMEA output.
Its worth a try anyway.
You may also need to play with the Advanced NMEA output settings to turn off all the superflous garmin proprietry sentences that your VHF will not understand.
Again - this is only what i would try after looking at the manuals i downloaded - i dont own one of these my self. Appologies if youve allready tried this...