OK, so we know the internal compass works, so let's disable it and connect the external.
With the external compass connected, connect the APM to Mission Planner.
When the parameter download completes, point the front of the aircraft North.
Watch the Compass Heading and rotate the aircraft Clock Wise (East) 90 Degrees. The heading should move towards 090 degrees. Now rotate the aircraft Counter Clock Wise (West) 180 degrees. The compass heading should follow the front of the aircraft, pass through 000 degrees and move towards 270 degrees.