When I first bought this car it had the leaks and such and it smelled musty and moldy inside. I noticed it had an oily reidue in the floorboard behind the driver seat. I steam cleaned it out and used damp-rid in porous bags to draw out the rest of the moisture while I fixed the leaks. Well the oily residue is back in the same spot. As far as I can tell it is oil of some type and water. What if anything would be in this area that would seep oil into the floorboard?

 

When I bought mine about a year ago, I noticed that the floorboard behind the drivers seat was wet after it rained. Although I kept the car in the garage most of the time, occasionally I would drive in the rain. I searched this forum and found that there was a design issue on earlier models that was fixed by drilling a hole underneath the Logo sill plate. It should have been done already on my 05, but I pulled the plate anyway to check. Sure enough the drain hole was there. Well, in my procrastination to put the plate back on, I had no more wet floor when it rained. I figure that the double sided tape used to affix the sill plate was just placed in a way that still funneled the water into the car. I haven't put it back on yet & still have no water leak issue. I think that there was a service bullitin about this issue. As to the oily texture I have no idea, mine was just water.

 

Sounds like it could well be that - the oil is being washed off something, I guess.