This guide was really useful debugging what I now think is a dead rad sensor. Just as an amendment, when testing the fan the guide says "In the rad sensor plug - red wire is stage 1, thin red/black is stage 2" and the common is the other. It actually appears that the solely red wire is the common and the red/white is stage one and red/black is stage two.
This is corroborated in this Bentley diagram (thanks Stonejag)