I think I know why...
I have a good idea how moisture is getting back there...I did all the custom work and posted a vid on how to remove your back seat. In the back there are 'pressure relief' vents in the back. They aren't the heaviest plastic vents so they open with a simple door closing.
It could be quite possible that they are allowing some type of condensation into the rear of your seats and since there is a 'back drop' behind the seat you would never see it unless you removed it like me to install an amp...it now makes sense why there was some buildup on the amp terminals....
Check out my vid and pause it at 1:37, you will see a small black shadow just to the right of the amp..there is your 'window of oportunity.'
<iframe width="425" height="349" src="http://www.youtube.com/embed/KeUcR0faIMw" frameborder="0" allowfullscreen></iframe>
Hope this helps!, And I'm Going to try it out and see if tappin' it will get it goin again!