Inner Wheelhouse Corrosion

[AC11]

I just picked up an 06 GX470 back in November. Frame is in truly great shape, 130K miles, no corrosion bubbling around the wheelwells. Fast forward a few months, I'm installing a backup camera and routing the wires through the rear trim and I find my rear floor insulation is soaked. I'm thinking the usual suspects, rear windows & maybe sunroof, but I pull the insulation out to find that the piece of sheetmetal that the third row assembly bolts into (I believe it's the inner wheelhouse) is corroded straight through. I have a couple of questions:

1. Does anyone have experience with this? How would this area maintain such a high moisture level to corrode through layers of sheetmetal? What is on the other side (the road side) of this piece?
2. Am I right in thinking that this is the inner wheelhouse? Lexus Part
3. It appears that this is a separate piece of sheetmetal than the rest of the inner structure body - can a body shop pull the affected piece, swap it out, apply some silicone, or am I in for a whole sheetmetal patching job?

 

[Batameez]

Look up a shop in NJ called Garden State Undercoating - this is right up their alley. A lot of body shops are too lazy to touch those jobs.

 

[JAGI410]

This is more common than you'd think. Mine did this too. Patched it up from both sides, added fresh undercoating in the wheel wells, and went on with my life. I have zero plans to use my 3rd row seats again, however if that's in your plans this needs to be professionally repaired.

 

[CarGuyCarl]

Mine has been in Chicago its whole life and it is fairly rusty for sure. I've got this problem too, discovered it while power washing my wheel wells lol