It's just a common thing with flat rear aerodynamics. Wife's hatchback 2011 Fiesta does it too.

 

The other day I saw a stylin' purple Scion shoebox (the one with no curves whatsoever) with dual exhaust exiting in the middle.
It had a greyish black pattern spreading up from the centered exhaust to the upper corners.
It was about the worst hatch dirt I've ever seen and must piss the owner off something fierce.

Never open the back door windows of an old style econoline van without opening as many of the side windows as possible.
The exhaust used to be like a hose from the tailpipe right into the van in my old 69 ford conversion van

 

I always thought it was like snake oil (a waste of money) until I tried it, but the window treatments like rain-x or aquapel really do actually help.
They won't prevent the window from getting dirty, but the wiper takes everything off much easier without streaks or scratches. Well worth the $15

 

The wind deflector is the only thing that can help. Back in 2006, Lund had one listed for the HHR. When I contacted them about it, they said it was discontinued. Doubt it ever went into production. Haven't looked for one since.

 

The deflectors are not only a bad thing for MPG but the bad air is also a problem for wind noise and viberation.

I have seen issues with bug deflectors where they have made noise or created aero effects that damage or tear off cab visors on vans and trucks becase of a vibration they created with the air. They pull throught the metal.

The same could happen on the back. The sheet metal is not all that great and I could see it pulling through is not reinforced. Not to mention the possibilities that it could make unwanted noise.

 

No one from SoCal who has never seen snow said to remove it?

 

I too believe that the spoiler made it worse, but it's really no biggie for me. I use to pull a trailer for work all the time before I got promoted, and switched to the HHR. I rarely look in my rearview just out of habit.

I will say though, I keep the RainX windshield fluid in my car at all times, I rarely have to use my windshield wipers as long as I hit it a couple of times on a day that looks like it's going to rain, and the back window, when I do use it, spray some of that stuff, the wiper wipes it off, and I can see out of it fine, normally without having to use the wiper again...

 

I use the Rain-X washer fluid too and it works.

I was hesitant because Rain-X didn't work well when it first hit the market about 20 years ago.

They've come a long way since then.

 

Anybody notice the generic washer fluid freezes these days? Never used to be that way.

 

The ElChepo blue stuff starts to get slushy just a little below 15 degrees and have had it freeze solid in the little lines several times.

I started using the yellow stuff when it first came out and then switched to orange Rain-X when it came out.

Don't know if it works at the advertised -25 but had no problem at -16.