94 Coachmen Class B
I ran in to this problem on my 95 van when I got newer door panels. I yes figured it out and finally just dremelled the newer mirror switch hole to accept the older switch. It looked ok, not great but ok. It's deep in the Rusty build thread somewhere, probably on the first few pages.
Wait, I just reread your post. I see now you are talking about the actual mirrors not the switch. I'll leave the info above in case it helps but as for the newer mirrors themselves you can either cut off the 4th post or drill the door sheet metal out with a step bit. To accept the 4 post.
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2010 Chevy AWD home build