exhaust in the cockpit

I am getting exhaust fumes inside my 68 fastback. It seems to be the worst with the window down and if I really nail it. I have plugged all the holes in the thrunk and have trunk seal on. I have holes cut in the back panel for speakers with no speakers so I even tried to cover up those with plastic. My exhaust is tight with no signs of leaks. I have tail pipes on as well. Any ideas where this is comming from? What is the "vent slot" on the passenger side, just below the rear fastback glass for? The pass side has it but not the drivers side. I did plug that tooo buy the way.