I have to do a patch panel huh?

Mine (a California rust free) rusted the same way. Don't be surprised if you find a big blob of caulk/seam sealer down in that area. Water will just lay on top of the caulk, finding its way thru the pores. Might plan on putting a trunk extension on that side as well. Along with the caulk my body guy found a crap of nails that had been down there for a while.....