For it to be visibly "melting" after a drive, you MUST be able to feel the heat or smell a strong solvent.
Heck, you should be able to smell SOMETHING regardless of whether it's happening while you drive or where it's parked!
The only way enough solvent could be getting on your mirror and topper to melt them while driving is if it's LIQUID, too. You'd see the spray while driving, and could easily figure out where it's coming from.
A gaseous solvent would "blow away" before it could cause damage while driving.
I suspect you NOTICED the damage in FL. It likely didn't happen there.