I would use the advice CLubford00 gave you. I had one of those rear air deflectors installed on my last SUV (a '99 Durango) I worked great at helping keep the rear window somewhat clear (also blew dust off the rear wiper.) However, I just recently saw the rear air deflector Ford offers for the FEH and I didn't care for it. Looks like I'll be putting up with the dust on the bumper.

Rich