I had a similar thing happen to me. when i bought the car there would be condensation in it every morning!
there would also be a puddle of water in the trunk. the culprit was the pas. side tail light, it would leak behind the seal so i siliconed all the rear lights and no more water
yours sounds light it might be the seal that goes around the trunk or the rear panel light on the trunk. try sealing the gasket and it might work.