ok, quick update...

got the volti meter out this time which showed a small negative current one way (~-2v) and the other way round showed ~-11v ?!?!??!?!?!?! Both negative.. So after giving up on that, I clipped the test lamp to the battery negative and testing the wires, then the battery positive and both wires, and nothing. Then I thought about testing a few more wires around the plug for the cluster to the dash so i could tap a new live, so I clipped the end to the key (ign barrel is grounded) and poked a few wires. Various things lit up etc. Then tried the switch wires again and one bloody lights up!! Tried the other one, and nope.. So, that must be the live. Plugged the live in and took out the neg, held the test lamp to the other pin from the switch and pulled the lever, and the test lamp lit up!! Hurrah!!!!! So hooked the test lamp to the battery positive and put the neg wire on the switch and held the test lamp to the other switch pin, pulled the lever..... aaaaand.......... nothing..

But then that's what really confused me. There's power running thru the lines in the right combo. The circuit is complete when the switch is activated, yet the tail lights (completing the circuit) don't light up.. ffs..

So, I went out, bought some extra wire, and I'm going to run a tap from the switch to the rear brake switch and be done with it. It'll be ugly, but it'll work. Black wire tape wrapped round, and no one will notice