I recently picked up a new-to-me tow vehical. Has a power rear window, which is very cool. Manual says don't drive it with that window down because of possible fumes. Do you guy's think it's still a problem if I have all 4 windows open and the sunroof? Don't want to fry my family's brains (too late for me). As I type this I'm feeling guilty even asking the question, but it's so nice with all that air going through.

If all the windows are open, its not a problem. Its similar to having a camper canvas setup on a boat. If you have all parts on, except the rear piece, theres a vacuum or suction there that pulls the exhaust fumes back toward the stern of the boat. But, if you have none of the canvas up, the wind while underway keeps the exhaust fumes out of the cockpit.