The switch for the low coolant warning light is in the coolant reservoir behind the air cleaner. At the forward end of the reservoir, there's a fitting for a hose from the radiator, and below the coolant level switch. You might have to remove the air cleaner for access. The switch has a 2-pin connector and is held by a circlip. The switch looks like this -



Replacing the switch is straightforward, just drain the coolant to below the level of the switch beforehand.

The formal name is coolant level sensor and the part number is 124 540 02 44. ~$20 for Hella brand through Pelican Parts.

First clean the contacts to see if that clears the light.

Sixto
87 300D