Technical Passenger airbag isolator

[freddyokel]

So, I've got an airbag error. No lights lit but an error none the less.
The error occurs when I try to disable the passenger airbag (using the key switch in the drivers side cubby).
The yellow disable light does not come on and the airbag ecu shows '14 - Passenger airbag exclusion switch - Plausibility - Present'.
I've had the switch out and apart and it's clean as a whistle and switching between 100 and 400 ohms perfectly.
I'm wondering if this is similar to the problem that Mk2 Puntos get and if the fix (a loom with an extra earth) would be the same. However, I'm guessing the Punto loom won't fit as both the switch and the ECU are on the passenger side in the Punto.
Any ideas? The car is a '10 Multipla M-Jet Eleganza.

 

[freddyokel]

Slight update.
I've tried swapping the switch with a known good one and... exactly the same problem.
If the switch is kept in the normal enabled position then the error can be cleared and everything is good. If you switch it to disable then MES detects the change and so does the car as the yellow light comes on. But... switch back to enabled and it throws an error.