Yeah the axle fitting on the end is the easiest way to tell, the thin ones have a nice rounded cast looking fitting welded on whereas the thick ones have a squarer piece which looks more like extruded section simply butt welded right on the end there.
Regards your wiring Ian, the circuit is positive switched, the positive supply to the switch is the green and the switched feed from the switch is the green/brown.
This travels in a loom up the N/S of the transmission tunnel and to the brown bulkhead multiplug on the N/S of the car.
This is quite easily visible from the engine bay but it is easier to remove the glove box thing to view from the inside. Could be a bad connection here?
After that the wire heads into the main dash loom, but it will split somewhere because the wiring for the rear lights runs down either side of the roof seam inside the car behind the head lining.
First off see if you have a switched feed coming out of the inboard side of that brown multiplug.
If not, bingo, if so it's time to open up the loom and find where the two wires meet.
At the bottom of each A pillar there are multiplugs that connect the roof looks to the dash loom but because the feed had already split into 2 the fault must be before that split, which will be lurking in the loom behind the dash not far from the brown multiplug hopefully.
Most likely it will split in one of those grey coloured junction things.
Anyway, lets walk before we run so check out that multiplug
