Go to page 48 of your instruction manual.* On that page you will find that the NMEA input is the red wire on the "GPS" cable and the white wire is the NMEA output.* NMEA input on the radio goes to the GPS NMEA out wire.* NMEA out on the radio goes to the NMEA in wire on the GPS or computer plotter.* With NMEA 0183 remember that "out goes to in" and "in goes to out".*
If you don't have an instruction manual go to http://www.icomamerica.com/en/downlo...x?Category=249
From Garmin you could use a Garmin GPSMap 76 handheld, a GPS 17xHVS antenna unit or the GPS 18x hockey puck (buy the LVC version which has only bare wires on the end unless you plan to wire into a computer).* The GPS 18x is their part number* 010-00321-36 (LVC Version).
Most manufacturer's GPS will use the NMEA 0183 (or NMEA 2000 on newer models) standard and will have cables or adapters to allow you to wire directly in and out of the radio.