You may notice that some vehicles (SUVs, wagons, or hatchbacks) have a small air deflector mounted at the top rear. There are 2 reasons for those deflectors.
One is to subdue the dust and muck from water/mud from getting on the rear glass and two is to prevent carbon monoxide from rolling up around rear windows, tailgates, and hatch doors.
With vehicles like the above a low pressure area will develop behind the upper rear part of the vehicles and cause muck and CO to boil up around the rear glass when the vehicle is traveling at speed. An air deflector will disrupt that low pressure area and will at least tame this problem to some degree.
These deflectors can be purchased in the aftermarket and they’re easy to install. I’ve done more of them than I can even remember.
If you go this route try to use a small one that is subtle. It doesn’t take much and you certainly don’t want one that looks like a wing flap from an airliner. 