I wanted to rotate the mirror for a couple reasons.
1. The shape of the mirror follows the shape of the rear window image when the mirror has the flat side down. Ever since I got the car it seemed like the mirror was upsidedown for this reason.
2. I wanted to raise the height of the mirror on the windshield. I am tall, 6'4" my head is near the ceiling, and the mirror is just too low blocking a lot of my view.
Notice that the stem ball joint is not dead center on the back of the mirror, in stock placement, the ball is on the top of the mirror. By rotating the mirror around the ball, the bulk of the mirror now pivots and rests above the ball joint.
There is a key in the mirror ball joint that prevents from just rotating the mirror the way I wanted.
First to deal with is the compression ring around the ball joint. Seen here as a steel ring.
There's a gap in the ring, insert your flat blade screwdriver in there and twist to open it up, and pop the ring up off its resting spot.
When the ring is off you can pull the ball out of its socket.
And now you can see the key grove in the ball that prevents total degrees of freedom.
Here's a closeup down the ball socket, you can see the plastic "key" on the lower left jutting out that fits into the grove in the ball.
Dremel took the key off in a second, it's just plastic:
With that key removed, put the mirror back together, ball in socket, then snap ring.
Here is a picture now from the side with mirror rotated upsidedown:
Now, just install the mirror in reverse order, rotate it back onto the stump, plug wires back together, and snap the closure halves together over it.
Here's what I mean the shape fits the rear window better with the upsidedown mirror, even the sides of the mirror follow the sides of the window slope better:
And now the mirror is higher up on the windshield.
I can see a little more out the window underneath it.
Having raised the mirror, the rear window view is slightly modified too.
Now the center rear seat headrest is not obstructing the image out the rear window in any way.
There is slightly less vertical opening view out the back window, the rear window is "squashed" in the vertical dimension a bit.
But I never look back!!