There is an important qualifier missing in the discussion of plastic rear windows. All clear vinyl is NOT the same. Cheap, Asian clear vinyl is often lacking the expensive u.v. inhibitors that prevent yellowing and cracking. It may be both too soft(scratches) and too stiff(splitting when cold).
There is much better quality clear vinyl available. The incremental cost of the best is trivial in the total cost of a window replacement.
If you 'search' you'll find a lengthy catalog I wrote of the various brands available. The short cut is to ensure you get marine vinyl from a reputable boat upholstery shop.
Beware ,it is very difficult for the untrained eye to discern the difference between the junk clear vinyl that many auto upholstery shops use and the best quality marine product.Do it once, do it right?
start here;
Strataglass | The Clear Choice Alternative Solution to Vinyl