On mine, it appears that water has entered from the rubber accordion-type tube that covers the wires going from the dash to the door.
I don't see a way to replace w/o removing all the door guts and rewiring.
Any ideas on how to fix the cracked-up old tube? I was thinking to just use something to cover it....
FYI: same issue happens all the time on the 914s too...I've heard that there is a mulit-tool from harbor freight that you can use to scrape the stuff off lickety-split with the wide 'blade' tool. It's basically a power chissel. Once the old tar stuff is off, you can assess the damage and repair/treat the rust.
http://www.harborfreight.com/multifunction-power-tool-67256.html