How to re-attach this piece?

brian6pac said

Can't you just take the thing out of the door, weld it and put it back in. Seems like the easiest way to me.

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By the looks of it, that would require disassembling the inner workings of the door/window assembly. Definitely not the easiest way to handle this. There isn't a huge load on this bracket. A couple small tacks will work great.