BenzBoy8, I don't know why you were thanking those that replied. I didn't find most of their replies all that helpful either. I understood right away in your first post what you were talking about. Maybe thats because every vehicle around where I live has one and people have different names for them.

Anyyway, I have a '79 300 D and the block heater is leaking coolant (slowly). The fastlane picture (of my 300D and your SD) shows a threaded plug with a threaded insert heater element. I am wondering if you can get a large socket over this outer plug and get a ratchet and extension bar on it from unerneath. After 25 years of the heater element being in there in my case, I would expect some effort would be required. The fastlane one looks like it's made of brass, so maybe it could be easy to get out.

I can see the problem from above (with a mirror) because some time ago, I tossed out the non-operating air conditioner that would have been in the way.

Please post what you ended up doing. There isn't much stuff here on this topic. I am intersted in how it went.