That is correct. I had a 1963 ambulance that had the exhaust exit the back, and if the divider window between the cab and the rear compartment was closed, I didn't get a headache, but if it were open just a little bit, I would get tired and a headache. I finally realized that the seal on the back door wasn't air tight, and the exhaust was coming into the rear passenger compartment. After that, I always made sure that the divider window was open, and I always kept the drivers window open a little bit to get fresh air into the car. When I sold the car, I told the new owner about my findings, and suggested that he keep a carbon monoxide monitor in the cab of the ambulance.
Ford Explorers have the same problem, and they had a recall program to fix the problem.