Rust through body near rear wheel well (1 Viewer)

[styran]

See video above. Rust goes through the body
Many years ago, the car lived near the coast, making trips to Lake Tahoe several times a year.
Any idea what causes this? The interior is apart because of a sunroof leak from kinked front and rear drain hoses.
I am unfamiliar with body repair and am not sure how to go about fixing that.
I hope this is not going to be a super expensive fix...

 

[Super77]

Looks like the cavity just behind the wheel well? That’s a common rust spot if water gathers in the low spot there. Luckily it’s easy to access from both sides and it does not require matched paint or appearance finishing since it is mostly invisible.

The right way is to cut out the rust and weld in new metal, seal, paint and make sure the drain works. The quick way is to hit it with POR15 and Gorilla tape. How many zeros add up to “super expensive” for you?

Also, check the opposite side.