1980C3

Why is this such an issue? Is it common? what are the tell tale signs?

A C

It's very common on C3's. C3 coupes all have t-tops. The t-top weatherstrip starts to leak after a while. The leaky t-tops let water run down the windshiled pillar post. It's a problem because the windshield frame is a structural part of the birdcage, which helps protect you in the event of a crash. You can usually see it by looking through the windshield from the front of the car. There is a portion of the pillar that is not hidden by the pillar post cover. You could also remove the post cover, I think there is only one screw that holds it on.

AC