When i was installing my muffler, i cut my old muffler out but the cherrybomb glasspack i got wouldnt fit correctly so i had to drive my jeep to school one day with the exhaust ending under the passenger seat. I did notice an exhaust smell in the cab so it can come from multiple places. An easy way to find a leak is to use Seafoam, the injector cleaner. When you pour it down your throttle body or suck it up using your brake boost line, it creates white smoke out the exhaust for up to a few minutes. When it starts making smoke, look around your exhaust system and see if theres any white smoke coming from anywhere but the exhuast tip. Read up on the process of Seafoam before doing this if you arent familiar with it but its a good cleaner to run once in awhile anyways. It will not make smoke if you put it in the oil/gas, only directly into the throttle body or a vacuum line.
04 TJ Sport- Kicker System, Smittybilt XRC-8 Winch, Cherry Bomb 18" Glasspack, 3" Zone Hydro Suspension Lift, 33x12.50 BFG KM2, 15x8 Procomp 7069, Aussie Locker front Dana 30, JKS 1.25" Body Lift, 4 100w Hella 500s