If you get them, grab all the wiring, as that is what runs the hi/low through resistance. Perfect used ones on ebay run about $100 - less if missing the wiring or with cracked grilles.
I presume we are talking about the fan assembly....not some type of aftermarket "tape" on rear defrost that mounts directly the glass. The rear defrost fan assmebly also works on Cougars, Mustangs, etc.