The existing mirror is almost certainly opaque silver film on the back. You should be able to remove the film with razor blade. I would pick up a kick yard mirror for the learning curve.

The best way to cut the mirror if you need to is with diamond wet saw for tile. I've used my right angle grinder with diamond wheel under running water just use masking tape and razor blade for marking exactly where to cut.

(This will not work if the glass is tempered)

You can use mirroring film like what I've used to keep sun from overheating a room to have a 1-way window .

I've used cars that have a hidden display in the mirror for the backup camera and it's brilliant.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)