glad you have it working
NMEA-ve is not the same as Earth (although Garmin use the same wire) it's a different circuit hence the radio having a red wire for the purpose.
Just to be clear - I didn't say the red lead was the earth
Red lead is connected to the earth but it is, as Duncan says, a completely different circuit and Garmin are obviously saving wires by choosing to have that side of their NMEA ciruitry tied to the negative side of the battery !