How to mirror a rear view mirror?

I'm trying to make a rear view mirror by adding a backwards facing camera and having it draw a smaller rectangle on top of the screen, but I can't get it to show a mirrored imaged by "changing the camera matrix" as explained here:

What I get is a flickering double image (inside the smaller rectangle): One regular and one mirrored layered on top of each other.

I added the following code

var mat: Matrix4x4 = camera.projectionMatrix;  
mat[0, 0] = -mat[0, 0];  
camera.projectionMatrix = mat;  

to a brand new, empty script for a brand new camera, but maybe there should be something else in that script as well?

I'll have to admit I don't understand what is meant by:

"Note that this doesn't work if you just get and set the [0, 0] value directly - you have to copy the matrix, change it and then reset it."

Copy the matrix? Which is found where?

There is the answer:

If you can use a standard 3D Cube game object as your mirror surface, the fastest way is to set one of the cube transform values negative. This turns it inside out, similar to the UV trick.
The camera will then be upside down, so the camera transform will need to be rotated 180.

Another easier approach (and cheaper too) would be reversing UV map horizontally for the mirror mesh.

If UV Map is mirrored, then the texture itself would be mirrored too, and moreover without any extra CPU usage.

Here a good Video Tutorial for Mirrors with Reflection Probes

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