Clive....... I’ve found the best way is to open up the switch (split the halves). Undo all the connections in the head light. Then pull the wire bundles back, out of the head light, through the bars and off the bike. Don’t disconnect anything at the switch. The problem will be the outer black plastic wrap is totally crisp after all these years. It won’t come back through the bars easily, and you will likely destroy the sheathing. Apply some grease our wire lube to them, before pulling them into the bars.
In order to route the wires outside the bars, you will have to cut a notch in the switch housing. I use a Dremel tool with small milling attachment. You’ll probably have to re wrap it the wire runs with tape, shrink wrap or vinyl tubing......