Best way to stop your back box rusting out is to drill a small (say 3mm) hole in the lowest part of it, to allow the acidic condensate to drain out, rather than just sitting in it. With much less sulphur (now Americanised to sulfur- grrr!) in fuel, the acid formed is much weaker, so exhausts don't rot as quickly as they used to, and especially if your journeys are long enough to boil off all the condensate. Diesels exhausts last longest, as they get a good internal coating of oily carbon to protect them!
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