Here is another possible cause. In my old Coupe the area I circled in the below picture cause water to get into my cabin.
CHeck that hose and where the rubber gasket meets the metal. Sometimes the hose is cracked or the rubber gasket not properly sealed anymore. Especially possible if you car sits outside in the sun quite a bit.
THat would then explain why it's wet in front of the bulkhead behind the seats. Water will run down the roof line and drip down behind the seat. Leaks are tricky. Sometimes the water finds it's way into the cabin through the weirdest of places and then runs down somewhere, even though the leak isn't really right above it.