The bottom of my driver's door and the passenger behind it is completely rusted through. It is from water getting between the door and the frame when it runs off the roof and then it sits on the door sill and rusts it. There are no troughs to divert the water. I have had it freeze in there and make the door hard to open in the winter. Body work or making sure that its wiped out frequently and rust treating the sill are the solutions. For some reason the passenger side isn't affected. At least in my car. My wheel wells are not rusted so it can't be from splashed up water. It comes from above.