are you sure that its a 7 pin cable? is the color of the jacket black or grey?
a 7 pin cable uses this color code and has a grey jacket.
white Nmea out +
blue Nmea out -
yellow Nmea in +
green Nmea in -
Red 12 volt output
Black 12 volt ground
the 6 pin uses this configuration and has a black jacket.
white Nmea out +
black Nmea out -
yellow Nmea in +
green Nmea in -
edit: nevermind, i see you got the right connections made.
make sure that the you have the sentences turned on. to do this you need to find the "advanced Nmea output setup" page, and make sure that both GPS status, and waypoint/route are turned on. look on page 74 of this pdf for more info on turning the sentences on.
http://www.garmin.com/manuals/GPSMAP...nersManual.pdf