Here is another possibility that has to be thought of.

How were the microfiche stored? If they were just in an envelope with gas and cardboard oil containers in the trunk there may be another problem.

They were not figuring that they would be unearthing a waterlogged car. So there is a possibility that the microfiche is unreadable, or simply destroyed entirely. Again I say this as I don't remember reading how the microfiche was stored.