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I'm not sure what the actual terminology is for this part. But I have a lot of frame rust behind this metal bar: https://imgur.com/a/XSsK5DP

It looks like it's not meant to be removed, so I'm a little worried since I don't have access to a welder right now. But there is a lot of rust behind that bar I'd like to remove. What should I do?

If you can remove that piece you will likely need to access it from inside the car. I'd take out the plastic trim around that area and look for a bolt.