My left front footwell was getting wet from water leaking in high up under left end of dash, just forward of door opening. Water runs in where guard, door & panel below windscreen all meet.
Also had some coming in where firewall & floor pan meet on right side - from water running down engine bay side of firewall. Also small amount coming in at heater.
There is zero rust in these places on my car, but old brittle sealant was the problem. I had to have hose running on windscreen while I was inside to find exact spots
Cheers, Gregg