It's a doublewall design which is why they rust. Total trap area for dirt & moisture to get trapped. Since basically nobody rustproofs anymore, this will continue to be a problem. In a few weeks when it's hot, I will pull the rear wheels, the rear wheelwheel liners, & hose these areas out to flush any dirt & remaining salt away. Let everything dry out in the hot weather, then slop a good layer of Texaco rustproof formula in these areas, along with reducing the stuff & spraying it, then button it all back up. It will never rust there after this treatment. My '17 has to last at least 10 years for me.