On my 81 300CD I had a trim shop cut a circle at the front of the center arm rest, then remove the foam rubber inside and install a cupholder from a back pack I found at an outdoor hunting/fishing store. My arm rest was always down and I only needed one holder. Never had to worry about shutting the door with an open container.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.