I had the rear spoiler off the IronMan when putting the satellite radio antenna inside it and notice some corrosion on the mounting bolts. It wasn't into the roof sheet metal, so I just cleaned the bolts with a wire brush and lubed with some Mobil-1 synthetic grease to coat the threads real well before re-assembly.

If the fasteners corrode enough it will spread to adjacent material susceptible to corrosion. That's likely how it starts, as the quality control on bolts and such can sometimes be poor as they come from a vendor and aren't an Isuzu product.