I thought the problem was exhaust fumes in the car? A blocked breather won't cause that as far as I know. It has been known to result in blue smoke from the exhaust pipe, but this isn't the complaint. I agree that if your breather is disconnected you need to connect it, but if this is the problem I'd expect oil fumes, which smell different from exhaust. I'd focus on the advice to seal all body openings at the rear of he car. As Jay mentioned, there is a low pressure area there that can cause exhaust to be sucked back into the car if it isn't airtight. I've heard that some have had good results with extending their tailpipes a few inches to get the exhaust gases out of the low pressure region. It might be worthwhile rigging something up temporarily to do this, and see if the fumes go away. Then you'll know where to focus your efforts. My 2 cents...
-David