This is a problem on many vehicles with opening rear windows or doors. I get the exact same problem if I use my D3 with either upper or lower tail door open. I find the best solution is to put the blower on max and direct the face vents at the face. Redirection of the exhaust tail pipe doesn't appear to work as the standard D3 exhaust tails face the floor, so fumes are directed well away from my tail gate. The only cure for this is to increase the air pressure inside the vehicle, keeping the fumes out. The heater blower is the only way to do that in my Discovery.