Leaks are a nightmare to find, sun roof is most likely culprit but door seals can be a favourite as well.
I saw an episode of Wheeler Dealers a while back when Ed China pinpointed a leak by putting a smoke machine in the car which showed a leaking door seal. He fixed it by sliding small pieces of rubber tube into the air holes in the seals.
I wish I had seen it a couple of cars back, I gave up on it eventually and traded it in.
Smoke machine is probably not that easy to come by mind you, do any of your mates enjoy a cigar?