From what I can tell by reading the manual, the 1987 300D does not apply any power to the glow plugs after the car has been started.
There was a design modification on the later 603 diesels (I'm aware of the W124 ones anyway, like '88 and '89 years) that kept the glow plugs on after the engine starts (up to 17.5 seconds in 1988, and up to 60 seconds in 1989 & later).
My 87 300D tends to be cold-natured on startup. A bit of rough idle and unburned-diesel smoke. I think it bothered the previous owner quite a bit, and I can see how it could be embarassing in a subdivision neighborhood. Fortunately (well, in my opinion!) I live further out and I don't care if the car smokes some for the first minute. I did have one bad glow plug when I got the car, and replacing it helped the rough idle at startup some. I suspect the other five might be getting weak.
Are you asking your question because of these 87 300D startup characteristics?