2005 Ford Mustang Why do water get in on floorboard

I'm no mechanic but chances are you may have a busted heater core. Either way the problem needs to be addressed, if you dont want to spend the money to have it changed, I'd disconnect it and plug it off, if not it'll rot the floorboards. If I were you I'd remove the passenger seat and pull the carpet back to see if the floorboard is wet. I had that problem with a 1988 mustang that I bought. If that isn't the case, you may rustholes in and/or behind the cowlbox