The dealer will laugh behind your back or right after you walk out the door. They do not put much effort into making sure these POS's are watertight, especially underneath and around wheel wells where you can't easily see. Out of sight, out of mind.
Get some caulk and foam, crawl under and start sealing anything you see where you think water could be blowing in. It's going to take some time, and you'll never get everything. It's like chasing a Unicorn. The Coroplast underliner is more for hiding the shoddy workmanship than it is for keeping water out.
