To help with visibility using the mirror thru the back window......
Be sure to fold down the head rests on the back seats.
Adjust the mirror to only look out the top half of the window.
If you adjust the mirror so the bottom of the mirror lines up with the bottom of the window instead of half way up you can't see as far down the road behind you as if you only try to use half the window. ..
Visibility is still horrendous, but can be improved. ..
Also, If you adjust the side mirrors so the you can see the side of your car, you have huge blind spots. The proper thing is to have them set wider so that as a car passing on the left is no longer visible in the rear mirror it appears in the side mirror and as it disappears from the side mirror it appears in your side window. Same for the passenger side mirror, but it's a little easier because its convex. Properly set up there is NO BLIND SPOT, even without using the "blind spot mirrors" build into the side mirrors.