The air flow through a car will be controlled by the pressure of the air around it. It's hard to be definitive about this because it depends on the shape of the car, its speed and also whether windows are open. However as a general guide the pressure will be high at areas like the front where the air hits a flat(ish) surface and low on the sides and roof, where the air flows smoothly, due to Bernoulli's principle.
As it happens someone has actually measured the pressure around a Honda civic as it's being driven. Actually they find the pressure at the rear of the car is hardly changed.
However a truck is a very different shape to a Honda Civic, and the air flows will be different. Also if you open the front windows the reduced pressure there will tend to make air flow out through the windows and therefore suck air in from an open rear door. The article I linked doesn't say whether the windows were open or closed, though i'd guess they were closed.
It would be interesting to see whether opening and closing the windows made any difference to the air flowing in from the rear.
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If the rear of the car is effectively a sealed box then I think you're just getting turbulent mixing. By this I mean there is no organised flow through the rear of the car, but the random turbulence at the open door inevitably mixes the air in the car with the air outside. I doubt you're getting much of the exhaust coming into the car, but the nose is very effective at detecting small concentrations of exhaust gas.