So this is a random observation I've had with my Gen 4 Prius 4 which I bought late July. I drive a lot - already have 8900 miles on the car - which I love by the way. Whenever I wash it (usually I got to an automated wash as I live in an apartment) the back end always ends up with drips and streaks. The dual rear window really causes some issues with its angles and such. If you washed it yourself it would be easier as you could towel dry but I still think it would be hard to get perfect. Certainly not a deal breaker - just a minor annoyance I have noticed.
Do you use the 'Touchless' auto wash? Before I traded my 2010, I was fearful even the soft wash brushes were going to tear the rear deflector off the car. I always carried a soft towel and dried the two rear windows because the blower never got them dry ... and as you said, they streaked and looked terrible.
I wash by hand, and never have had a problem. The only "rear end" design fault I've noticed is that the rear wiper only works on the upper glass, whereas the lower glass (acrylic/perspex actually) is the part which gets dirty. And, yes, they aren't going to put a miniature windscreen wiper on it either.
I've always had to stop by an island and grab some paper towels to dry off the rear (Gen 2 and Gen 3). The fans never catch it and it doesn't dry the rear.
LOL, popping back and forth from the very highly technical supraforums to this site is mind numbing due to threads like this.
