You never mentioned what year of engine type you own, but as far as installing the thermostat at the highest position in the housing, it's not all that obvious for some models.
The housing is nearly level on the 3.2 M104 engine configuration, but if you check verrrrry closely, you can see a slight dip near the back of the housing.
You will be 99% correct if you have the little vent hole pointing toward the windshield, which, if you are looking directly above the housing from the front grille, is at about the 12 o'clock position...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car