It seems a lot is riding on the quality of the undercoating, since any breaks or cracks will let water in and then it can't get back out. I have been hitting the dry areas with either spray cans of undercoating or truck bed liner to seal it up a little.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma