MB 300D's from somewhere in the late 70's to 1985 were W123 chassis and all of them should have interchangeable grilles. The 240D's of almost the same period to 1983, I believe, should also have the same grill. As far as relay and glow plug, you will need to look and see if the wiring has separate wire going to each plug or a wire going to firewall end plug and heavy zig zag bare wire in between the rest of them.

Almost all of the interior parts of the W123's are interchangeable as long as the color match is not a problem. Front seats are also in this category. Some had black plastic switch panels where the fancier 300D's had Zebrano wood, and there are different positions for various switches in the panels with different years and models. Pays to have pictures or diagrams with you when you are a newby, and sometimes even if you are not.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150