I just found that the dust cover is not completely gone. It fell down.

Since it seems to be made of thin fragile plastic, I want to be careful.

What is the best way to put it back?

Just snap it in with some force?

Or use some adhesive?

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.