Every so often (6-8 months) the rear wiper on my 99 Explorer stops
working. I can hear the relay going in the back and I can also what
sounds like the motor engaging.
While it was under warranty I had no problem asking the dealership to
fix it. Now that it's not, I'm going to bother to figure out why it
keeps happening. It seems like it's a bad design for it to repeat.
I sort of think it has to do with some connection between the motor and
the wiper arm becoming worn or loose. Is it possible (I wouldn't rule
this out, personally) that slamming the rear door shut could do the damage?
Does anybody have any thoughts or experience with this?

Thanks,
Ambrose