The rear of the car has a swirl of air caused by the lower pressure. The air rushes in to fill the the gap so to speak. This sucks the exhaust gas into the same area. If you don't have proper ventilation in the cab to increase the pressure then the exhaust fumes will be rushing into the lower pressure of the cab. You can see it happening on a dirt road. If you drive a ute on dirt the tail gate is always dusty. If you put the tonneau on then the back fills with dust. If you undo the tonneau and fold the rear up under the rib you get no dust in the back as the air pressure is equalised.
Unless you sealed the tailgates shut with sikaflex I think you would really need the fan going to pressurise the cab.