I agree with strips of paper to find non sealing gasket.
Chalk powder or charcoal powder is also a good detective,
pour on gasket, close lid, open and look for pattern.
Another way is to put a high powered cordless light in trunk,
do this at night, in a dark place, the leaks will glow !
Rear window is a possible culprit, soak outer seams with soapy
water, use an air hose from inside, poor seal = foam