If you can strip the internal panels / carpet off to see metal panels - draw a horizontal chalk line, test with hose and the trickle of water will wash the line away.
Once found, work your way upwards till the source is found.
Mine had damp problems due to the hatch not being aligned - Golf had a random trailer light connector in the boot made me assumed that a PO had a bike rack on the hatch, weight of bikes pulled out the hatch at the top.