I live in upstate in New York where it snows lot. I have two xb's one 04, one 06. In both rocker panels partially rusted out. I am beginning to think that a major cause is the covers--trapping salt and dirt to the undercarriage. I bought the 06 used and it was immaculately well maintained and thought I could escape rust with frequent visits to the car wash during the winter. I go to diy wash places and focused on the underbody. But recently I have removed the covers to discover significant rusting out. Do the covers trap salt and direct it to the rocker panels? I have spent some time on the 06 fiberglassing the rotted out rocker panels. (One nice thing is that the cover covers to lousy job I have doe in this fix). I have closed some of the rocker panel circular holes and remounted the cover--thus there is a way for water to flow out of the space between cover and panel. I will report in the spring if this seems to reduce rusting by flushing out road salt.

 

That would not surprise me. On my son's 06, we had to replace the plastic inner mud guard and had to get into the front rocker panel. Not much rust as it's an AZ car, but there were a lot of trapped pine needles in there. Aleppo pines, 6 inch long needles, horrible trees, should be banned.