You have to take off the complete door panel (but you probably are aware of that) and if I am right, the original armrest has plastic pins sticking through the door panel that are melted on the back side to stay tight. Normally you could screw the armrests to the door panel after they are loosened...no need to buy a new armrest...

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)