I have been lurking and picking up tips for some time. Maybe I can contribute. I replaced the thermostat on my 420SEL today. I got to the hidden bolt with a 9 inch 1/4 drive extention (12 inch would have been better). Do this by going behind the alternator, between it and the timing chain tensioner, above the oil filter. I do not know if the stud with the small gauge wire was hot so I put a piece of heat shrink tubing on it; of course you could disconnect the battery. You can feel around with the socket on the extention, it's not too hard to find and remove the bolt. To install the bolt, I put the bolt in the socket and was just barely able to feel the hole with one hand while inserting the socket/extension with the other. With the bolt in the hole I removed the socket so I could start the bolt with my fingers out of fear of cross threading. It is very difficult to do this, you probably can't if you have large fingers.
The top bolt actually gave me more trouble because none of my wrenches would go on it and there is not room for a socket and ratchet. I took a 10mm cheap box end wrench and ground down the entire outer edge to make it thin enough to go on the bolt.
Hope this helps someone!
Harold