I’m not sure what you have or don’t have on the back of your car, BUT on my car every morning its covered with dew, I live close to the coast, as soon as I get on the freeway and up to about 45 the water streams start to clear my rear window, by the time I hit 80 that rear window as well as the rest of my car has been wiped clean of the water. Now my car doesn’t have a spoiler, nor does it have the bulbous stock wing. I'm running a wing off an old 86 firebird. Now in the rain behind MY car you can see what a difference the wing makes versus what it was like stock with nothing, the road surfaces water vapor looks like its sucked up behind the car then shoots upward off the wings rear lip obviously creating some downforce. As stock with nothing it was like you said, my back window never got wiped clean by the airflow. That’s just my car though, all wings are not created equal. For those of you who think funny little flat surfaces don’t have much effect on your car, do a little test and teach yourself something about downforce. Get someone to drive you while your in the passenger seat, get going about 60 mph and stick your arm out with your hand flat, tip it or down very little and you’ll feel just what downforce is. See even a small surface such as your arm and hand can create some serious downforce. Now have your bud hit 80 mph and see what happens when you stick your hand out the window like a wing. Now go 100 mph and YES many of you hit that on a regular basis now go ahead if you think you can and stick your hand out the window. So next time someone tells you wings and spoilers are just for looks, this or that, or they don’t contribute in any way ask them to verify that opinion by sticking their arm out a car’s window at 100 mph.