Ok so...

Terminals 1 and 2 act as they are supposed to.

#1 is showing around 5.04v until the button is depressed, then down to zero.

Pin #3 has around 500mv.

Pin #4 is where my issues start, I do not have any voltage and grounding it does not illuminate the light on the switch.

I have just bought a new multimeter from Jaycar and it has some strange induction thing that i don't understand so my 500mv may have been a false reading of zero. The car has gone to the movies now with the missus as I am about to leave for night shift, so re-testing will have to be done tomorrow.

Could a faulty light lead to a zero reading? Could this lead to the diagnoses mode not activating correctly?