found my wiper shafts rusty, uhm, must be H2O getting in them, noticed that they had probably been removed since factory, and noticed that the shaft bushing only stuck out of the body sheetmetal a little bit (no extra threads exposed past the big chrome nut) this made the shaft bushing top BELOW the well in the body that the shafts stick out from. Thus water collecting in the well, would overflow down the wiper shaft, causing it to rust, and getting water in the trunk, which lead to the felt under gas tank getting wet, causing gas tank corrosion (estimate 1/3 or so thickness reduced in rust pitted area on tank exterior that was touching the felt pad).
So make sure your wiper shaft bushings extend up past the top (water level) of the wiper shaft wells in the body, else your shaft will corrode, and your trunk will be damp, causing further corrosion problems elsewere.
how may you raise the shafts if need be? well the shaft is externally threaded its entire length, it is positioned up and down by two nuts, one on either end, that secure the threaded shaft to the wiper frame. By lossening the bottom nut, and backing it off, and then tightening down the upper nut, the shaft will then protrude further thru the body (also need to loosen / re-tighten the chrome shaft nut)
it might also be time to renew, or at least add a dab of silicone RTV under the rubber shaft seals to prevent H2O leakage around the shaft bushing outside.
good luck, stay dry unless you bathe of course.
ain't really old cars fun?