The armrest in my 1993 300E is broken, too.
Is yours the kind that has the flip-up lid with the storage compartment?
My armrest will now only stay in the fully down position, I can lift it all the way up, but it used to be able to be adjustable in many different positions.
I'm told they are very, very expensive from the dealer.
I hope they can be fixed...
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".