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I have seen them either way. I put mine on the inside corner b/c the outside seemed like it was taking too much away from my mirrors. I bought 1 of those "JDM" broadway mirrors that clips onto the rearview. I love it. It's wide as hell but I can actually look at it and see out the entire back and out the pass. rear quarter window. I can see myself and my passenger as well. You can get them in crazier sizes. I think mine is like 280mm.
Here is an easy way to figure out. Just put a little tape on the back of it before you remove the 3M type stuff that really adheres it. Put the mirror on the inside then the outside and look and see what helps you best.