Mike, not that it's going to resolve your existing problem, but recent batteries I have purchased from our local MB dealer are sealed. So there is no issue with leaking battery fluids. I would be reluctant to cover any wiring with POR-15; however, I would take the time to clean some of the connections and wiring insulation. The wiring situation is not desirable, but it does not appear so far gone that a full replacement is required.

I doubt that heat from the exhaust header caused the other problem area.

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Fred Hoelzle