Someone elsewhere posted that Rainex shouldn't be used on Acrylic...would be fine for glass though. I remember doing a search some time ago for a rain-ex like product that was for aircraft windshields which are usually plexi/acrylic. Can't recall the name ATM though...might search again later if I get some time tonight.
As for the ferret, I never fed it the soap, it ate it. Amazing how resourceful those little beasts can be when they want something. It also loved shredding neoprene. I had an old wetsuit hood it got ahold of and started on the neck bib...took it away, put it on the top shelf of my closet, 7' off the ground. About a half hour later I saw the wetsuit hood 'running' across the carpet again, being towed to the favorite under-the-bed hiding spot. Turns out the ferret had figured out it could gradually leapfrog its way up a set of wire shelves to the top (stand, grab with front legs, curl body up to grip underside wires with back legs, scramble around the lip with liberal assistance from teeth into the rubber coating on the wire, reposition and repeat...), scurry over and grab the hood, and then just 'jump' and survive the fall. Suicidal but determined as hell, they are. 