Steering Wheel problems of the 2007 Toyota Tacoma

I bought this truck new and purchased the optional anti-corrosion system at the time of the sale. At the 2-year check required by warranty, I noticed corrosion in the vicinity of the rear bumper and frame. The dealer, atkins-kroll Toyota on guam, submitted this concern to the maker of the system, who denied any responsibility because I used the truck to haul scuba diving equipment. We have subsequently engaged with atkins-kroll directly, and they have failed to fix the problem, claiming they need to consult with their headquarters to determine if this is covered under warranty. Meanwhile, my bumper has corroded through on the right side and separated from the frame. The majority of the corrosion is in the bumper and a crossbar at the rear of the truck bed. All the hardware in the bed has corroded as well, even though I fresh water rinse my truck bed every time after I haul dive gear (I have done this since purchasing the truck). There does not appear to be any appreciable corrosion beneath the forward portions of the bed, just the back. A fellow dive instructor with a 2005 Tacoma pre-runner experienced the same thing, and his bumper completely came off. I understand earlier Tacomas (I believe 1999-2004) were recalled for similar corrosion issues. I am convinced this corrosion is due to a materiel defect, a design flaw, or both but I have not been able to get conclusive action from the Toyota dealer to date. When I contacted Toyota's corporate headquarters they said they could not help me, that I had to work this out with the dealer. It is just not right and, more important, just not safe.