The block heater is located on the passenger side of the engine block, tucked away up under the manifolds. I took out my old one because it wasnt working when I got the car, and installed a new one. I did this without removing manifolds or anything else, so you can do this, but access is tough. It is one of those situations where you barely have room for the swing arc of a wrench, so you have to do the routine of moving wrench a little, flipping the wrench, or using a couple of tools to advance the thing, etc. etc. There are two versions...the older one which is larger....larger diameter threads, which fit directly into the threaded hole in the block. The newer version has smaller diam. threaded portion, thus requiring a reducer ring to be installed into the block before the actual heater, which threads into the reducer (I'm not sure if the reducer ring comes with the block heater, or is already installed in the block with a plug). You need to get up in there and see what you have first, then order parts etc. etc. I installed the older larger version (which I had to order from MBZ), and I had to buy a very large socket to fit over it (I couldnt find metric for cheap, so I used a 1 13/16" from sears). Because of the large size of the socket, it was difficult to get it in due to limited access. I assume that the newer smaller version would be a little easier, except that I assume that the reducer ring would still have the same 1 13/16 size to deal with (unless it is already in the block). Im not sure of the details, but this is definitely doable by a DIYer. Well worth it....works great....when I go out to unplug in the morning before leaving for work, I can hear it sizzling in there, and the engine is warm to the touch only after about 3 hours (I have it on a timer...goes on about 4 am).

good luck

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)