Thanks missfixit70. I am now running a small two core wire across for left hand tail and left hand turn. I presume I don't need to also pass the ground across. For that matter I dunno why I have to pass the left hand tail across (maybe there's a switch to make the tail lights operate as indicators ...

Ah, the box says "Lift roof", so I guess it's a red herring. Moreover, all the lights operate without it plugged in, so it rules that out.

Maybe there is no left hand set of wires also available on the right? Do they really split at the front and then branch either side of the vehicle all the way ...

I've fitted a towbar to a Ford Freda and am faced with the cable from the towbar, a little piezo sounder with half a dozen wires, and the right hand side panel removed...

There's what looks like a relay with two plugs going into it. One with four large wires and another with about two dozen small ...