When I purchased the car ( a field car) it had an aftermarket ( square) mirror, so it was weld the holes or install a correct one ( that was REAL expensive).....

If I remember right ( I am getting old) as odd an install as it seems, outside the pass door vent glass is the only place it works.. Outside the Pass door glass, it will not move enough to work for the driver.. Again its been a while, but seems like a lot of hand wringing ( outside the vent glass) when I did the install.. Pretty sure I looked at some other cars ( computers were not as common then) also.....