You need to remove the wood panel near the door opener first. Push up the bottom of that small wooden panel and pull it gently out. You will see a screw behind that which will make it possible to take out the next panel that contains the trim along the (un)lock button and the power seat button containing trim.
Behind these trims there has to be a screw that holds the door handle...there may be some additional screws in the underside of the armrest. You have to remove the chrome trim near the lock as well.
I never went any further, so I don't know what fixations are in the back of the door panel. You need to shove it upwards or pull it towards you after which the top of the panel has to be shoved upwards to get that part out...

Somebody will probably give you some more useful advice...

greetingz,

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