I'm in Northern California. Not much rain and no snow where I live, and the car is in the garage most of the time.
On Edit:
I took my truck into the dealership this morning and showed them the rust, it turns out it is on the chrome. they are going to replace the chrome unit above the license plate under warranty. They did say that they don't replace too many of those, they don't see much rust on 4 runners
I took my truck to Autowest Toyota in Roseville, CA
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I just picked up the truck, they replaced the chrome piece and they said that the chrome was too tight against the door and it had pinched the foam that goes between them and the two metals were touching. They replaced both the chrome and the foam and hopefully that fixes everything, the good news is that they fixed it AND it is now documented that it was repaired under warranty once already. Not that I expect it to happen again, but better safe than MAD (uh, I mean sorry)
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06 SR5 V6 4WD Meteorite Metallic