Floater inside reservoir has a little magnet on inner sides. And the sensor from underneath is practicaly just a reed contact.
There are also two versions of function, depend on car make:
1) Normaly open logic - When fluid level is OK, floater is above reed contact and they are open - light is off. When fluid is low, magnet on floater drops down and makes reed switch conductable - light is on. If you disconnect the wire, light will be off.
2) Normaly closed logic - When fluid level is OK, floater is high enough to reach reed contact and magnet shorts them - light is off. When fluid is low, floater and magnet drops, reed contact goes apart and light is on. If you disconnect wire, light will remain on. So it is reversed logic, no big deal to do it with electronics.
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