The sender is attached to the coolant reservoir tank. It is black, residing near the front lower area, just below where the minimum fill level should be. Wires attached to it will give you a clue as that is the only electrical part attached to the tank.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car