The Icom units, whilst excellent, are a little fussy about the individual data sentences within the NMEA data string.
Specifically they need a NMEA 0183 GGA data sentence for positional data. I tried to hook one up to a Lowrance unit that was only providing a NMEA 0183 GLL data sentence, and the two would not talk. (GGA and GGL are two different formats for the positional data).
Most modern GPS should output GGA data strings, though my slightly older unit would not.
Thread from sat nav news group here: