I have this problem in mind to introduce to the list.  I don't remember ever reading about it.
If you look at the end of the gutter on the Vanagon at the rear, you will see that it does not extend to the very end.  So, the water leaks down into the car through the vent, right down to the engine area, into that pan on both sides of the van, and rusts out seem, depending on which way the van is leaning when parked.  I have never seen a pan that had its hole clear to drain the water, yet the channels are there to collect it to the middle.
Now, I propose an extention to that gutter to dran the water by the rear gate.
One more to improve and save.
Zoltan