Inspired by another question I wanted to ask about phenomenon that I've experienced.
We had a van with separate trunk deprtment (similar looking one below) to transport things. But some of the things were too big, so we couldn't really close the rear door (the door was actually opening up, not like in the pic before) so we left it partially open (some 30-50cm gap). That's why I had to get inside and hold some boxes so they wouldn't fall out.
My experience felt like a gas van. The smoke from exhaust pipe was going straight in through the open gap. Why did the smoke have to go through the trunk department? I've also heard similar stories of friends driving a car with open trunk door - smoke seems to flow inside and through all the car. 
Answer
Your car is perfect for the generation of turbulence by flow separation. Behind the car, the flow cannot follow the car contours. You get circulation patterns on many scales (i.e. a lot of large and small turbulent structures). Naturally, there will be pressure differences at the opening. Consequently, there will be an exchange between outer and inner air. The exhaust gases are being circulated upwards around the rear of the car and become part of this exchange. If there is a mean inflow through the lower part of the opening, there will be a circulation within the car and the whole cargo space will be filled with exhaust gas in no time.
Here is an illustration of the turbulent flow around a car: 
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