I had mine exiting out the rear, then modified to exit in front of the rear tires after I lowered it. I'd say you have either an exhaust leak or a window seal leak that's allowing fumes to get in through the back window.
Mine has a factory bumper exit. No real exhaust fume issues except when you're just idling in one spot with the rear window down.
You will find that there was never a factory exhaust that vented directly below the tailgate. Even the old Volvos 240s had a different final pipe. If you want to know what is and is not safe, start your car on a chilly morning and watch where the steam goes. Where it goes, the exhaust gasses follow. Even on sedans, I make sure to run the outlets to the outboard side of the tail, so I don't suck exhaust in when carrying loads that leave the trunk open. A roof rack air deflector helps as well. It washed the back window in fresh air plus helps keep road grime off the glass.
If my windows were up I had no issues, with them down, definitely! My exhaust dumps out under my car right behind the muffler (to give the axle plenty of room to travel), and it makes my dual Flowmasters sound pretty awesome so I can deal with some fumes.
There's a reason that it's called the "station wagon effect" for all vehicles really. When the exhaust fumes cyclone up in the negative space behind your flat glass. We have side wind deflectors on our Valiant that do a pretty good job of eradicating this situation as well as blowing the dust off from every day driving like MikeT mentioned. Rear facing exhaust doesn't make too much sense on a wagon where you're constantly dealing with the 5th door and choking yourself out.
