Failing open seems to be a common failure with these thermostats (guess that's better than failing closed!). Last winter (around 0 degrees centigrade is about as cold as it gets here in winter) I noticed our 300TE taking forever to warm up, and only then reaching less than 80. A new thermostat fixed it. I have done several others with similar faults. Testing old thermostats against new ones in a pan of water on a stove reveals that they still appear to function, just at a lower temperature.

That temperature gauge tells a lot about what's going on. All of ours normally sit just above 80 (I estimate around 85). Only in heavy traffic in our Aussie summers or climbing a mountain pass heavily laden on a hot day will the gauge get up over 100.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.