Piper Archer II Air Leak Problem by Back Seat.

Sounds like a door seal that isn't sealing well. A door seal leaking,
especially along the back edge of the door will, suck air out of the
cockpit (same is true of the pilot's storm window, if you've ever opened
it in flight it creates a considerable suction, in fact there is a SB
that requires a placard warning not to open it over 150 MPH because
doing so creates enough suction to actually break the stock 1/8"
windshields). Being that it is the right side that has the draft,
rather confirms this diagnosis. I have a similar issue with the rear
left seat in my six. The door has a gap at the rear bottom and that
sucks cold air in from the screened air exhaust vents behind the back
seats making that one seat pretty chilly.