I have a crack in my original catalytic exhaust and I seem to be getting fumes into the interior. Someone told me that maybe the rear tail light gasket was the problem as the lights are cracked and the finisher is warped. Could this be true? Not much of a mechanic.

 

it could be true

 

I think it is the "crack in your Original catalytic" that is causing the Fumes!
put on a hi flo cat or a test pipe.get rid of the cracked exhaust.