[Paulporsche]

If you pry off the center cap with a small pick or screwdriver inserted into the indents on the outer ring (knob), you can then remove the knob by holding it while pushing aside the inner steel retaining ring with a screwdriver. I think it goes counterclockwise but it it should be apparent which way to go.

Once you remove the housing you will see how these things work. There are 2 fingers that mesh together to hold the back in place. They may be worn, or you may find you can otherwise tighten the whole mechanism up.. Mine worked fine when I redid my seats so I didn't mess with it.