100% agree with the comment from Dave Porter.

I would opt to get my money back if that is an option. The dealer never should have sold a safety certified car to you that was “rotted”. The mechanic who certified it could loose their license.

I would have a change of mind and tell them you want a refund. I wouldn’t want to be driving a car that had a rotted frame, unless the frame was completely replaced, which, unfortunately, would be cost prohibitive considering the value of the car. Too risky to weld it, how would you ever verify the structural integrity?

By the way, I also live in Ontario.