I bought one of these for my 1977 240D...It's been a while but I believe that it goes in the right side of the engine very near the starter.

If I remember correctly, I used a large adjustable wrench to get the plug out. It wasn't easy, but I finally got it removed. The I used some teflon tape to cover the threads on the new block heater...about 3 times around and screwed it in. Connect the plug to the heater, wire tie the cable out of the way and plug it in. What a difference in cold weather.

It's a dirty job and you'll get wet...not so wet if you drain the coolant first.

I believe that the one I purchased was 400 watts...get a timer...you'll only need it on about 4 hours in most weather...you can run it all night, but you'll notice it on your electric bill.

Good Luck
The Tenor Man