I just removed the cassette storage from my '90 SL - I actually have removed the entire dash to replace a leaking evaporator. The cassette holder is removed in an entire assembly with the ash tray, and then it slides out of the bottom of the assembly.
You have to remove the radio and temp controller, then remove the wood console cover, the armrest and the top of the console. When you get it out, you end up with a nice black plastic box that would hold a cup holder nicely - before I put it back in I need to find one that fits.