I'm really surprised this topic has so few comments?
This is my first T4R and I love it except...since day 1 the coolest tailgate window that stows had me amazed. Until I drove down a dirt road and my new black on black TRD Pro was seriously full of dust in 5 minutes.

Then the rains came and winter roads just spray the back window beyond any logic of any vehicle I have ever driven is insane!!

I researched the laws of aerodynamics and learned there is a solution that move the low pressure air further back away from vehicle as the high pressure is compressed and speeds up. Ironically this reduces drag at the same time somewhat like winglets do?

I work in the industry so I am going to have my friends crunch some numbers with the width, length at low velocity to create a proportional deflector or surface, stay tuned for the progress reports - unlike Honda - Toyota doesn't have aerodynamic engineers or concerns which I understand given the application- but they are driven on roads above crawl control speeds