Without a doubt, replacing a thermostat on my E420 last Christmas was one of the most aggravating jobs I have done because I went into it thinking, how hard could this be? I mean - really? Grrrrrrrrrr
Combined this job with a coolant system drain, belt replacement and a Duovalve changeout. Fully committed by cutting off the short angle hose using garden shears, which was later a bear in itself to get the new one back on. (Had to boil the hose to soften it and use silicone grease to slip it back on - hated to do that, used as little as possible grease on the interior of the hose as to not contaminate the coolant system.)
Had to try 3 different socket - rachet combos before finding one that (barely) worked to put back the 10 mm capscrew on the opposite side of the garden shears. Settled on a tiny 1/4 combo set. Don't even think about trying to get a torque wrench on these capscrews. Another reason to use the 1/4 mini set.
TIPS - This job required removal of the serpentine belt, and removal of the oil dipstick capscrew tube to engine block bushing
to access everything, particularly the back bolt. (Second photo with oil tube pushed away.) Another lesson learned, remove the engine oil dipstick and put something in place to avoid dropping anything down into the engine. I kept the dipstick in place and wound up breaking off the plastic red head with one bump. (That piece of wisdom cost me $29, and I never found the red head that rattled down.