It's probably a fluid level sensor. It's just a bare contact touching the
liquid. You need to read it with ac. Set up an oscillator with a gate,
feed ac to the sensor through a cap, and rectify and meter the ac voltage
on the cap. Compare this with a threshold for ok/nok switching. Since you
measure 20K to ground a series resistor of about 68K will be required from
osc output to capacitor. This should swing the o/p voltage from about 25%
to almost 100% when the water level goes too low. The frequency can be set
low, say 100Hz.

hope this helps,

Peter

PS: I am almost certain that the radiator should be grounded somewhere,
unless it's aluminium, in which case it probably should not (to avoid
electrochemical corrosion)!

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