One way to determine if the air is coming in from the tail, tape some yarn to the inside of the turtle deck hanging down, either set up a mirror or take someone agile enough to turn and look in the baggage compartment for a flight,
I had allot of air coming in when i first got my plane, i took a shop light and stuck it into the cowl then looked for light on the firewall from inside the cabin, i found 2 places where light was clming from the engine compartment on the edges of the firewall, i sealed it up, problem solved