There is a post here on change the door strap under the DIY section ....
DIY 124 door strap
What is shown is for a front door ... but the rears are similar. Once you get all the pieces off, remember that you are going to lift the panel up. The inner door panel is simply held on by both the top edge and the plastic hooks that hook into the metal door itself. These hooks break very easily so life gently ... your total upwards movement is less than an inch.
Also the area which holds the inner door latch will slide back and then out after removing the metal arm that attaches to it.
I am sure you will find either a loose bolt (10mm or 8mm). Also check the door panel itself. Sometimes the parts of the armrest come apart.