Had the same problem -- hit a pigeon who was on the highway looking for something to eat, and wiped out that access door (not to mention the pigeon, RIP).

Got one from the dealer for a relatively high price ($50-60 if memory serves) but Phil at Partsshop could very likely get it for you for less.

I took the liberty of using some plastic 'wire ties' (you know, the toothed nylon strips used to hold bundles of wire together) to keep the new door in place since they have a tendency to come off of their own free will.

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Steve
'93 400E