Saturn cars seem like they have lots of leaks of water into places where it should not go. This page deals with water getting into a Saturn Trunk (or Boot if you are British). As the cars age the seam sealer that Saturn used to seal the seams between different metal parts of the car hardens. When the sealer hardens, small holes open up allowing water to go places it was not intended. For the Saturn car trunk, the common place the water ends up is in either the driver side or passenger side wheel well of the trunk. If you have a sunroof on your Saturn it might also be that the rear drain outlet is not secure against the body but most likely it is the seal between the metal bodywork.
To fix the water leaking into the trunk:
You should now have a dry trunk the next time it rains.