This is weird....anyone help?

The coolant level dashboard light came on recently and won't go off. I've checked and topped-up the radiator, and replaced the sensor. Still the light stays on.

I think the sensor has a magnetic reed-switch in it as I can hear it "tick" when a magnet is placed nearby. Both the old and the new sensors give the same (apparently working) resistances when tested with an Ohm-meter and the magnet.

I figured it might be a seized magnet-float in the radiator, but I'm sure it's not that. This is because I tested tried the magnet near the sensor whilst it was connected, but not housed in the radiator. Still the light stays on.

So it must be between the sensor and the dashboard (I think!). Any ideas anyone??