Resurrecting this thread for any help getting that bottom tab into the slot to lock the cover in.
Installed the mirrors a couple weekends ago, passenger side was perfect and easy. I was really pleased with the fit and finish. But I ended up breaking the bottom tab on the driver's side, even though I knew exactly what to do and what to avoid!
Got a replacement cover and I still can't seem to get it in the slot. The bottom slot seems like it is at a much more acute angle, I can't angle the cover into the slot like I see in this autoid install video (at 17:50 mark):
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(FWIW, these are the Autotechnic ones sold by IND which look identical in every install I've seen).