My cup holder failed first under warranty and was replaced by MB. As soon as my warranty was up, the cup holder failed again. I removed the assemly, found poor designed plastic retaining tabs for the spring had broken. I replaced the spring retaining tabs with the thin metal piece (size of a letter return label) and reinstaller the cup holder. That was 2005, and even with lots of use and no special care in use.......never a problem.
Don't let any debris fall into the cavity the cup holder rests in. It can jam the holder. In my case, this causes the spring to break. I was able to rig it so it worked again. Later the little gear that allows smooth deployment became loose and wouldn't let the holder deploy. I had to remove the assembly, push the gear down to get the holder to extend, then put the holder back in place while already extended. When the gear finally broke completely off, the cup holder began to work properly! :)