Cheers, I thought as much. I continued on and have disassembled the instrument cluster down to the PCB. I have multimetered IKE X11175 Pin 21 (Airbag) to ground and see there's 10v on it. The trace runs straight from the pin to a transistor, from the transistor to a resistor and into the left side of the LED. I've got 10V at the left of the LED to GND (Pin 1). Funny though, I've 10V at the right of the LED too, there's a 1001 resistor joining the two sections together. I'm thoroughly confused.
I'm going to swap a known-good LED to the airbag tomorrow evening after work.
Meanwhile, here's a diagram.
2013-12-09-00.22.26.jpg
The red line is the trace from Pin 21. I've 10.8V all along it. I multimetered all yellow spots to Pin 1 (GND), all gave me ~10V. I was not expecting voltage on the other side of the LED, the further right yellow dot which is negative. There is no voltage across the LED itself (yellow to yellow). This confuses me.
Perhaps stranger still is that the convertible roof diagnostic light in the purple box lights up fine, has voltage across the LED etc but it shares the space with the airbag negative too :huh: