If I'm remembering correctly, around 43-45 farenheit the plastic window undergoes a 'glass transition' which is to say the usually semi-malleable polymer loses it's compliance and flexibility, in the same way a summer tire turns into a solid hunk of rubber in winter. The plastic becomes rigid, and therefore brittle, raising the risk of cracking instead of folding. This condition gets worse with age, as the plastic 'dries', losing it's flexibility and yellowing. A friend, trying to bang snow off the roof of his Miata, shattered it's back window, which had already begun the UV ray depletion, was yellowed and stiff even in summer. I'm not aware of any products that prolong the life of the back window specifically, but plastic cleaner/polisher like Meguires kept my previous convertibles' rear window as clear and healthy through-out the 5 years I owned it. It outlasted the top.
BTW ToolPants, I don't feel stupid doing anything in front of others in my Boxster!
Even crashing probably looks cooler in a Box'.