I removed my cassette holder and installed the storage insert mentioned above. (129 680 09 91) It actually works pretty well as a cup holder even though the hole is rectangular... not ideal as a cup holder, but a decent substitute!

I thought I read somewhere that someone was offering an aftermarket cup holder that fit in there, but can't find it now.

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)