The GM B/C-cars (Impala, Catalina, etc.) were the WORST of that era for lower rear window rust issues. The 3M product "AlumaLead" came out about the time it was needed for these cars. At least down here (TX) the Chrysler Fuselage cars didn't seem to have that same level of deterioration. I suspect it was due to the fact that Chrysler probably put more paint in that area? When you consider that any car with a similar rear window/roof contour situation is also subject to water collecting in the lower corners, as the pinchweld is almost 1/2" below the outer sheetmetal surface and will collect water/condensation there.
When our '66 Newport was about 7 years old (I took it to college), I was messing around one sunny afternoon, in a dry portion of the year, and pulled the rubber trunk mat up just to see what was there. What I found was CONDENSATION drops on the bottom of the mat! Plus small amounts of water puddled in the depressed ribs in the trunk floor! It looked like the paint in these areas was heavier than normal, and more haphazardly applied. I looked for obvious weatherstrip issues and found none. I determined it was from the hot/cold cycles of the climate, as the car had NO history of any water leaks back there. The rubber would not let the trapped moisture evaporate out, so "trunk carpet" of the more expensive models had a double-duty purpose! When I got my '70 Monaco a few years later, with trunk carpet, no such condensation was found in that area.
The mat in the '66 Newport was thinner and more "solid" in nature, so any moisture under it would go nowhere and grow with time. The mat in the '67 Newport was more "molded rubber" and a little more porous in nature. With age, it got hard and would break when moved. I planned on replacing it with carpet later on, but it still hasn't happened yet.
These were my experiences.
CBODY67
