I'm glad you at least found the problem!
1) How does a large rust hole appear in the top of a tank ?
2) How does rainwater get in, in large quantities, through the mesh grills in the engine cover, and on to the top of the tanks? It obviously does, in large quantities, and rots tank tops. Also it gets on to the cylinder head, and sits when #1 and #2 spark plugs are, and rusts them in place.
Maybe we need to cover those air vent grills when the car is sitting not in use.
As an aside, when you open the engine cover, if it has rainwater on it, that water pours on to the cylinder head.
Plus when you open the front cover, water pours on to the radiator fan, with later models having an ally L plate over the fan motor to direct the rainwater away.
It seems Europas do not like rain. Most doors leak, and old windscreens leak and rot the wooden dashboard.