Cause of condensation in trunk
Leaking top. Heat of the day will cause the water to evaporate and with no where for it to go it collects on and around trunk lid and linings as condensation.
How to find and resolve leak.
Only way I can think of is to open trunk and apply water to the top and watch for the leak. Something else to think about -- did you happen to let the top down with it wet? Or have you placed something wet in the trunk?