[drcoastline]

You say cowl? but it appears to be the rear seat pan? This is a very common area for these cars to rust. I can't say specifically why they rust in this spot, but they do. Don't worry about finding the cause. Your car is 36 years old it will be at least another 36 years or more until it needs to be dealt with again if at all.

It is a single layer of sheet metal at this location. That spot looks significant enough that the best course of action would be to cut it out and weld in a repair piece. Also, the rust in that spot is most likely several inches larger than what you see. On my car I purchased the entire seat pan section from a car that was being parted out and welded in the repair patch. The rust on my seat pan looked very similar to yours. I went a little larger and included the brackets from the donor panel in the repair.

After welding in the patch epoxy prime the metal and undercoat it. Your car is 36 years old, that spot is not all that bad considering its first thirty-six years were probably far harder than its next 36 years. During its first 36 years it may have been a daily driver, probably sat outside often, was driven in the rain, maybe snow, etc. Now and moving forward it most likely will be garage kept only taken out on the best of days and may not see any water even when taking a bath. Rust moving forward won't be all that much of an issue.