You are missing some wires!
The VHF should have (plus power etc):
Blue (NMEA IN +)
Green (NMEA IN -)
Grey (NMEA OUT +)
Brown (NMEA OUT -)
(Convention would suggest black should be grounded but something is odd with what wires you have)
The 55dV has 4 wires
Brown NMEA IN +
Blue NMEA OUT +
Black GND POWER AND NMEA
Red POWER
So... Connect:
VHF BLUE TO Garmin BLUE
VHF GREEN TO Garmin Black
VHF GREY TO Garmin BROWN
VHF BROWN TO Garmin Black
You'll need to check sentence settings (probably OK), NMEA Mode (Garmin's often default to Garmin's own cr@p make it NMEA 0183, Baud Rate usually 4800 unless sending lots of data like AIS.
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