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Try running water over the rear area of the car with a hose. Make sure the inside roof of the trunk is dry before starting. If there is a leak you should be able to spot it by seeing where the water is sneaking in.

Check the seals around the trunk.
You may have alos left the trunk open a bit by accident.

However, as mentioned above if you have warrenty (bumper to bumper) take it in because of the electrical issues that came around.