Just a note on why it can be the earth connection...The two circuits have separate earths in the headlight, so if one of them is more corroded than the other (depending on which is touching the headlight casing), then that can cause a loss of connection. Like I said previously, it's exactly what happened in mine with the only exception being I had dipped working and full beams not working.
Ben, it may not be necessary to test the earth connection if it's obviously corroded (covered in a bluey limescale sunbstance). If this is the case, simply clean it up with some sand paper or even a pen knife blade will do if necesssary.
Having said all that, if it's easier to do initially, it's probably worth swapping the connections on the switch though as suggested by Oliver, that'll confirm which end of the circuit has the problem.
I'm not familiar with your headlight fixings. If you need to slacken off the headlights, then mark on a wall/fence where the full beam is roughly pointing and you can use this marker to align the unit when it comes to putting it all back together again.
Good luck!
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1998 VX1600