Just did my '87 190E's wiper blade insert recently. Having removed the wiper from the arm, there is a thin stainless steel strip running the length of the blade on each side, which pull out (with thin-nose pliers) once you've released each strip (each has a notch at the bottom end, just use a thin flat-bladed screwdriver to persuade it out).

Pull out both stainless steel strips, then the rubber blade itself can be pulled out. Replacement is reversal of the above, though the rubber can be quite tricky to guide back into the arm - be careful not to damage it!

Oops, I meant this to be a reply to Jim Anderson's 'Wiper Blade Insert R&R' question, not for it to be a new thread by itself - D'oh!