And Chris said:

I disagree with the latter.  Without a point of reference you will never
know if your mirror moved out of adjustment until it's (BAM!) too late.
Besides, my Westfalia has a nearly useless rear-view mirror due to the
protruding rear A/C unit.

Chris.

So I said:

If you put a different internal mirror on the windshield, I use a small 8"
from an AUDI, mounted so you can see out the rear window and also use a
Fresnel lens on the rear window you practically eliminate the rearward
vision problem.

pensioner

89 Westy Syncro with pendulous AC