[jclark003]

your RF ground (or drain wire) typically is connected to a sea water ground plate. So some source that has direct metal contact with sea water.

Chartplotter issue sounds a little strange. So your ships icon moves backwards? So if you are heading North the icon moves South on the chartplotter, therefore having improper reference to position?

The position and movement on your chart plotter is usually a reference to the boats position (gps) and heading is in reference to where your ship is pointing. Is it possible your chart plotter is in North up mode and you are heading south? then your ships icon and movement will be oriented downward on the chartplotter.

Basic check of heading sensor is to compare it to your compass heading. If close then it is probably not installed backwards. If you want to calibrate your heading sensor check your Garmin installation manual for how to enter the initial setup/sea trial menu. There should be a calibration option. Most have you drive the boat in circles for a couple of minutes.