Hello Dave,
i modify a 3 positions for mine in that case you need also modify the center part to have the four positions, and you need have another "scrap"switch to ad another plot on the 3 positions, for cabling your it's not easy without a good reflexion, i could make you a drawing of what you need check in all the four positions.
on the workshop manual drawing the switche is in the position "only tail light" generator to tail light, and earth with resistance connexion, you need check with the 4 positions face in place, in all the positions.
maybe more simple to explain with a drawing , i make this this afternoon for you.
all the best
Arnaud
