Sorry if you have done this already but the Icom M505 is slightly different from what the Icom’s usually are.
The there are two wires stuck to each other next to the power cables, split them, the one with the white writing on in it (closest to the power cables is a two core coax type wire, again spilt the two cores, the plain now unsheathed cable is the neg and the inner is the NMEA + in.
Also with the Garmin go into the set up menu, go to the network/ NMEA options and make sure NMEA network is set rather then Garmin.
Hope this helps.
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