I have a 97 saab 900s 3 door which water leaks into the trunk on passenger side, just right of the spare wheel well. i found that water leaks in even when the car isnt being driven.

Had the same problem with mine. The water leaks in around the passenger side taillight assembly. The gasket doesn't forma good seal and therefore you have to "glue" it back on to the car (make sure you clean all the dirt out beforehand) with silicone adhesive or RTV. Hope this helps.

Check for any rips or tears in the rubber seal around the lift gate, also make sure the rubber seal has not came loose from the lift gate. Then check the mating area for any damage.