RockyRoad

I would say it's more likely the way the inner and outer arch skin are joined. They're just spot welded together so aren't fully sealed then you have the arch protector rubber thingy that probably holds water that runs down the inner arch then finds its way through the layers. I don't remember any sealant between the layers on the saloon arches I removed a while back but even if there was, it doesn't last forever. Or it could be both and there's other spots in there that the "open" side of the layer is facing up so water can creep down inside.

NOSSY_89

I should have clarified, its not just the black vents causing the rust but it is one of many contributing factors. Lets be honest the Japanese did skimp in quite a few areas so they could compete in the market. How many 90s cars had electric windows or other equipment at the price point they were selling at. Saving had to be made somewhere without cutting into their margins.

RockyRoad

I think you've hit the nail on the head. These were a lot cheaper than the equivalent German car right? That cost saving has to come from somewhere and of course it's going to be somewhere people aren't going to see when it's new.