Garmin typically uses a black common ground in the NMEA wiring, that would be RX- or TX-. The VHF uses independent grounds. This sometimes creates confusion in connecting the wires. It's just a matter of getting the two manuals and checking the wiring connections, matching up the color codes, and making the connections. As others posted, make sure the data you're trying to share is going to be recognized by the equipment you're connecting! If it's AIS, make sure the chartplotter can use the AIS sentences, and insure that the port you're sending it to is the correct baud rate. Typically AIS is ported at 38,400, there should be 2 NMEA ports on the chartplotter, make sure you're using the ports that the baud rate is the same, or it'll never work. You must RTFM for this stuff, if you start randomly connecting wires, you can damage the gear, can be fatal for the NMEA ports. Sketch it out, make sure your colors are correctly matched, observe the polarity, and turn the gear OFF when making connections. If you plan to integrate other gear, it may pay to install a terminal strip to make the other NMEA connections accessible for future connections.