I've seen folks with something on their back window, looks like a plastic
rectangle that sticks on from the inside so they can see better.  Does this
really work?  I don't know what it is called.

Is there a preferred size?  Also some put in dead center, others more towards
passeger side. ..which is better placement?

Where do you buy?  Do they really stay on or can they get moldy as some that
I've seen are brownish around the edges.