park it somewhere dry (and safe, obviously), leave the hatch open, strip everything out of the back that you can, like the floor, side compartment covers etc, disconnect the battery and then just have an air dryer blowing it out over a day or so. there are threads on how to check the drain pipes for blockages and how to unclog them. your problem may be less related to a perished seal than to a blockage in one or more drain pipes.

good luck either way

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