I was going to replace the t-stat in my 89 420SEL. I noticed that the service CD showed 2 different types of housings. One on top of the water pump and one on the side of the pump(passenger side). I assumed that the T-stat was on the top.....so I removed the upper radiator hose, and removed the 3 bolts and the cover came right off. But my T-stat wasn't in there!
It's on the side of my water pump which will require a complete draining of the coolant and it looks like I'll need to remove the alternator too to get at it. I'm just going to do it next year when I replace all belts & hoses. So don't be surprised if you take off the top of your water pump and the t-stat isn't there.