In theory to the subject at hand
the wagon i just got has not much rust but i did have a look at the rear tail light section today
as i was interested to see for myself
mine is a 71' model i did notice that there was a bit of rust on the left hand side around where the tail light seats
but the only thing i could think of why they would rust there over time is the rubber seals that seal the tail light to the body where buggered or deteriated over time due to mosture sitting there...
other than that i'm sure there is no design fault,

hope you find a nice wagon got to love them....good luck..