did similar with miata e-mirrors (pic)
use the oem mirror gasket as a template,
make sure the mirror head can fully rotate for breakaway before you drill holes
trial-fit - you will likely have to grind a curve into the mirror base for a good fit
be aware that on 911's the right side is set back more than the left
- especially an issue for 914 -, else your vent pillar will mask the view
911 right templates are on the internet
you also want metal reinforcements inside the door under them, else they will eventually crack the door panel
.
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