I have a 1983 280ZX 2 seater with Ttops. When I am inside the car even in heavy rain there are no obvious leaks. However about every other time it rains really hard the seat belt on the drivers side and the seat back is soaking wet. Now I have set in the car for an hour in the hardest rain and it had no leaks. Ive tried getting it to leak in a self serve car wash and no leaks. But then I go out after a heavy rain and the darn seat back is wet again. ANy ideas?

Memphisheat

 

If the seat back is wet... it's the t-top seal.

 

It's probably because the water gets in in such a way so it cant spill without encouragement, for example it may pool in an area and not overflow until you go up or down a hill or something, my sunroof has the same problem, I bet the water is getting in and leaking down behind your headliner into the seat belt, if I were you I would take out the big thick rubbery (or whatever that stuff is made of) trim piece under the t-top gasket and see if it has rust on it and then remove the interior panel that surrounds the quarter window and feel around the little area behind the door plunger for rust or pooled water. This happened to me with my sunroof, only the water would flow toward the A pillar trim piece, luckily the trim piece was smashed at the top so water couldn't flow down it like a gutter into my dash.