80 series rear window wiper no go? any ideas what would be most common causes?

Hi All

We have a 96 80 series and the rear window wiper does not want to work.
I had a bit of a look at the wiring in the steering colomn to see if i could find if it was a faulty switch, but it got a bit confusing.

I checked to see if there was any power to the rear loom in the tailgate when the wiper is switched to on, but there was not.

There was however power in a single wire when the hwindow heater was turned on as WELL as in a twin loom, which had power in BOTH wires, (when i assumed one was + and one would of been - which gave me a thought of "bad earth"

But the wiring loom does not just run to the wiper motor, it has some wires comming from the wiper motor on to the heater / locking mechanism, which then leaves me confused again.

I read there is an earth up in the rear head lining which can give grief but not sure how to get into there without doing some damage.

There is also a box located in the rear tail gate with "wiper control unit" on it, which looks all fine when i looked in there but does have a slight "smoked" smell.

Then yesterday when Mrs was using the rear wiper suddenly moves up and then stopped in the up position??

I seem to get confused with cruiser wiring as they dont seem to run simple power in / out like old Holdens.

I know its a bit dzzy my info but I just thought there might be a common issue or a more common sense approach someone know?

Regards
GN