Wouldn't a warm trunk and cooler outside air cause condensation similar to what happens with a toilet tank? If so what is the source of the warmth in the trunk? I think it could be the fuel tank. The fuel is constantly being circulated thru the engine and warming up, thereby heating up the trunk. It would be interesting to compare your trunk with a similar vintage gas model to see if the same problem occurs because the gas isn't circulated up thru the engine.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13