Hi,
Sounds like you are not making correct connections from Garmin to VHF
radio. This is a two wire connection, Garmin NMEA out and ground.
What is needed is to have Garmin interface set to deliver NMEA out via
its' cofiguration menu. Then all that is needed is two wires from
Garmin to VHF : These two wires should be within a screened cable
between the units.

1 NMEA out wire (probably blue) Check Garmin manual!! and
2 Garmin ground (probably black)
and to connect these two wires to VHF NMEA connection.
If correctly wired the position will appear on VHF as soon as Garmin
finds its position.