That's a good call about the hood Pete... maybe that was the purpose of the neutral switch? can't think of any other use for it except maybe something emission related? I'm going to use it for a remote start.
Anyway, happy days, I've fixed the reversing light switch
I got the multimeter on it and no matter which position the plunger was in and which combination of wires I was testing, it was completely dead.
The switch itself comprises of 2 basic parts, the main body which incorporates the spring loaded plunger that screws into the tail shaft of the transmission and the switch contacts which are housed in a metal cylinder that's crimped onto the back of the main body of the switch.
Mazda dealers / vehicle technicians would test this switch and if it was dead would just bin it and fit a new one but we're old school home mechanics right? and a bit more ingenious than that ( plus I don't want to shell out £170 for a new switch).
So into the vice it went and I carefully hacksawed the crimp off it to separate the two halves of the switch.... only thin soft metal so it was done in a couple of minutes...
the two halves then pulled apart...
This revealed the components and workings of the switch which are simply 4 copper prongs that connect directly to the 4 wires coming out of the back of the switch...
and a long spring loaded plunger that is an extension of the plunger seen on the outside of the switch...
The plunger shaft just slides through the contacts giving 3 different signals depending on its position. It has plastic in certain positions that breaks the conductivity. On this switch the metal prong contacts were a bit dirty but the plunger was pretty horrible so I cleaned it up with an emery board, flushed the whole thing out with WD40, cleaned the contacts and put it back together and it worked fine
I then tried it in the transmission and again it worked perfect which means transmission modifications were also ok so that's great news.... the gearbox mod isn't difficult but accuracy is essential, especially when drilling the selector rod.... just a fraction out or if the hole isn't drilled in exactly the same plane as the existing hole and you end up with a box that's notchy or won't select a certain gear.
For future reference the pair of red wires to the switch( which are in turn connected to the long switch contacts) are the wires to the reversing lights..... the pair of green wires ( which go to the shorter switch contacts) give a circuit when 5th gear is selected. When the plunger is fully out that gives the signal for the reversing lights, when the plunger is fully retracted that gives the 5th gear signal.
I need to permanently reassemble the switch. There's no pressure on the bit that contains the contacts during the operation of the switch so I'll clean it all up externally with brake cleaner then Aradite the 2 halves back together.
Next job is to pull the tail shaft again, clean the surfaces again with my trusty brake cleaner and then rejoin them with some liquid gasket.
.
....... great to have it back on the road in the sunshine... 
............ so I'm going to make a couple out of some copper sheet.