Rear demister.

The rear demister in my Mk2 has rusted, corroded is probably a better word for it, It had little breaks in it and when i got the windows tinted it made all these yellow dots.

Does it having all these little breaks in it stop it from working?
And how would I go about fixing it? do I need to buy a rear screen thats in good condition from a wreck or can they be bought new?

Cheers