1.6 fan switch O-ring

The 1.6 Miata is unique (in the Miata world) in that it has a switch to turn the primary cooling fan on - it's not switched by the ECU, rather by a switch that goes to ground at a certain temp and turns the fan on. It's sealed by an O-ring, so if you have some leakage between the switch and the thermostat housing, you need a new switch. Here's that part, at a fraction of what the factory part costs. The right size, rated to 300°F, and specifically rated for coolant (ethylene glycol), this will last a long time.

Replacement for F8B5-18-T41.