Spoiler for Rear Window

[LsHart]

Looks like it would be way too much downforce. I vote no

Is it for a Nascar? Would be just for looks.

 

[Doubeleive]

OK, so you're looking for DIFFERENT aerdynamic effects, to keep your rear glass cleaner?
It ACTUALLY DOES get filthier quicker than the other windows, you're not alone.
That 'spoiler' definitely won't cut it. You want a Kamm-back, I think. Look at newer 'suvs' (yes I know most are too carlike, but hang on a sec).
They don't use a huge Kamm-back, but they do add some material surrounding the top and sides of the backlight to prevent air from passing directly over it,
instead of the air being attracted to the rear window and depositing dirt, it keeps moving backward, missing the backlight.
Hypermilers fashion much larger ones out of clear plastics to try to improve their fuel economy.
If all you want to do is keep the backlight cleaner longer, you really don't need a big one, it only needs to be a few INCHES, not feet.

This is obviously way too big, but look at 'cuter' suvs and you'll get the idea:

https://ecomodder.com/forum/member-unicycledan-albums-testing-picture6453-dsc00053.jpg

that kamm back design doesn't work for diddly sh*t, the rear k2 window gets completely filthy compared to my gmt900, very noticeable
I think the only reason they really did that was for "looks" and to house the wiper arm

 

[Marky Dissod]

that kamm back design doesn't work for diddly sh*t, the rear k2 window gets completely filthy compared to my gmt900, very noticeable
I think the only reason they really did that was for "looks" and to house the wiper arm

At the meager size of the K2's pseudo-Kamm-back, of course it's not going to work.
Freely admit that the size needed would not be worth the trouble for any OE manufacturer.