Own a 2007 Pontiac Vibe (Toyota Matrix). Anyway, do a lot of driving on gravel roads, and the rear window is always covered in mud or dust. Any type of deflector available to alleviate this ?
In the 60’s you could buy an air deflector for a station wagon that mounted on the roof and directed air over the back window. Did they work? Not very well as evidenced by the fact that you don’t see them anywhere anymore.
Your best bet is to fill up with washer fluid and use the rear window wiper and washer liberally as needed.
A pair of rear wheel mud guards might help a bit.
Use Rain-X on the rear window. It makes the glass slicker so that dust does not stick to it as well. Mud will be a little more difficult, but the rear window will stay clearer. It helps on the windshield as well.
I used to live on a gravel road and had a hatchback at the time.
Those rear window airfoils worked pretty well but they will cost you some fuel mileage…
Both .Caddyman and. Galant have good suggestions. As far as deflectors go, it is often very model dependent. But I found too, as Keith says using “rain x” window washing fluid, and plenty of it helps my SUVs most on the dirt road we live on. Get your own squeegee and don’t let it build up too.
thanks for the sugestions guys will try the rain x and I gues some type of spoiler if I can find one
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. 
At the risk of repeating myself or some one else, “galant”'s suggestion is quite important too. Larger aftermaket rear mud guards do help a lot. That’s where much of he grime is coming from.
I would get mud flaps, as others have mentioned. These are more likely to help than a spoiler.