Given the fact you say your exhaust exits well back and assuming it's all tight and leak free i'd be thoroughly checking all grommets etc on the firewall,actually the fact you drive with the window open most of the time may well be exacerbating the problem.
The airflow passing through the radiator will be picking up any fumes/smells etc and with the window open it will generate negative pressure in the cab pulling any fumes through even the smallest gaps in the firewall.
I was having similar problems but an afternoon spent looking found a couple of points,one was the heater hoses and a loose wiring grommet.
Having fixed them the cab is fume free and much more pleasant and SAFER to drive.
Last edited by jockbolter50; 01/28/2015 5:32 PM.
