i think they all rust there. the fresh air duct collects crap like leaves and pollen in a corner in that area, it stays wet, rust. i have fixed one (in primer) need to fix the other side. every one i see here in junkyards are all rusted out there.
as long as the repair is not massive, you can use panel adhesive to attach new metal ($30). cut out the rusted area, form new panels to fill the hole, make a "filler" and glue it all together, strong as weld.....no matter welding or using the adhesive, it's a pretty big job.
if you want to try glue, it's easy. lets say you cut back to good metal(yellow), then you glue the filler(blue) piece to that, then butt up the patch(red) with more glue, pretty easy.