>There is a float style sensor directly in the coolant reservoir.
My 1990 SC w/VMM has a dual-contact submersible probe that is attached to the top, extending down low in the coolant expansion tank.
The contacts rely on coolant/water to conduct electricity between them and if they aren't bathed in coolant, the circuit turns on the low level warning light - I've seen these become covered with deposits over time that can eventually inhibit current flow between them, triggering the low coolant indicator in the VMM. With our cars seeing so many years and miles, it should be no surprise that this type of component needs a bit of attention.
The sensor probe can be removed (pried out...note any index tab) from the top of the expansion tank - use caution, as the leads from the top of the probe to the contacts are sheathed with plastic insulation that can be easily cracked/broken (due to age/heat) - this can leave the leads exposed - suggest resealing with silicone. This could also be a good time to remove and muck out the expansion tank if it contains any sludge, etc.
Carefully scrape or wire brush any deposits off of the inside & outside face of each contact and then reinstall into the coolant tank and reconnect the harness. Fill the expansion tank to the right level and check for proper warning light operation.
If the contacts on the end of the probe are clean, check the circuit at the probe connector.