36 years of compression cycles......getting in and out of the car loads the sill plate. When the wire is hot (hot day in SFG), the wire insulation is soft. Eventually, the wire strand works its way thru the insulation. The sill plate or the fiberglass surface under it are not perfectly flat so there are higher load areas. Takes time but it happens. Back in the olden days, the Corvette fit wasnt the best either. In 76 and 77 they were building a SP full of them.