This episode was just on. The t-tops are on, and if not, they aren't glass, how can you tell. Secondly, the lights arent on. The sun just makes it clear plastic in front of the headlight clearer.
Is this really a "slip up?" If anything, it's more of a stupid thing to do. Why does Homer have Snowball (the first, seen in a flashback) fanning a block of melting ice sitting on their carpet?