The days of passive sensors in cars are over. Have been over for a long time, because this is the ABS wheel speed sensor used in 2004+ T5 VW transporters, and it has a bunch of elecronics inside (I’m guessing similar to this device).
If you try to measure the resistance, you will get ~4.5 megohm.
So, connect 12V to pin 1 (the flat side of the connector) via a 1 kilo ohm resistor, and 0V to pin 2. Multimeter to pins 1 and 2. You will see the output voltage (i.e. the current through the resistor) change when you wave a magnet around the business end of the sensor.
In my case, the sensor was not the problem.
This was the problem.
Replaced the connector end with a good one from the spare, and all is good.
(My word, people can gippo).