When wiring AIS into a NMEA 0183 port you would need to match up the NMEA version and baud rate. Checking the data in port monitor should result in seeing AIS sentences. These normally would start with a "!". If you have done this, then my guess the Garmin unit is outputting NMEA at a RS232 signal level and not a RS422 level. You might need a level converter to get a proper interface. If you doubt the NMEA port on your MFD; you can do a loopback test by taking data 2 output back into your data 2 input, then turn on a sentence like GGA and see if you get it back clearly. If you don’t get clear data, you might have a NMEA port problem with the MFD. Once you get the raw AIS data showing up in the port monitor; you need to "Checkmark" AIS on the input area of that port, so the unit will process it.