Connect Chartplotter/GPS to VHF Radio for DSC and position data

Garman to Lowrance. It didn’t work straight away for me with an oem cable joiner, so I cut the plugs off and found out why (NMEA RX (-) green cable on the radio needed to be tied to battery (-) and there was a small black wire that was (-).
Same as last one, boat, beer and dodgey filming on phone