You're asking that question six years too late.

 

​​​​​​I know this thread is old but I still see people searching for some of the problems. I had the problem with my floor being wet all the time on the passenger side and having some fuses blow from getting wet that are on the floor the passenger side. I discovered that the holes on the firewall are getting clogged with dirt and that's what causing the water to leak inside the car. Mine were full of dirt so I even throw a couple extra hole so it wouldn't happen again, but also check your cabin filter for a long time I didn't even know I had one which is up against the firewall pretty much behind the battery. I hope this helps even though this post is old I'll still see people searching for this and other common problems that the mustang has and they must have just bought their mustang