See page #3 https://www8.garmin.com/manuals/webhelp/GUID-D4F04D40-7D5A-4040-AD3F-27506E0DAFD0/EN-US/ECHOMAP_UHD_Install_EN-US.pdf
Brown and Blue on the power cable for NMEA0183. Blue is transmit from Garmin. Brown is transmit to the Garmin.
