88 IROC-Z, definitely getting exhaust leaking
into car. Don't hear or see leaks in pipes/mufflers.
Any "common" places to check?

 

blow smoke through the system from the y pipe connection back and watch for the smoke. oh, dont forget to put something over the rear exhaust so the smoke will not get out that way and build up inside. dark smoke - or red if you can get it. then watch for where it is coming out and that is your leak.