Further testing suggests that the water is entering the trunk under the lid at the rear corners just above the taillights. The problem appears to be caused by the lap joint where the rear wall of the trunk is attached to the rear quarter panels. This horizontal lap joint is visible about 6 " in front of the taillights along the edge of the lip on the trunk opening. When the trunk lid is closed, the trunk lid gasket crosses this lap joint. On my car, the lap joint is slightly depressed. When the trunk is closed, this depression creates a gap below the mating surface of the gasket. During heavy rain, water runs down the channel on the outside of the gasket and then slips under the gasket and runs down the inside of the gasket to the back corner of the trunk where it is blocked from draining to the outside by the gasket, so it pools and then runs over the edge of the trunk opening collecting in the wells behind the rear wheels.

Testing with smeared grease, the only way I can get the gasket to seal over this lap joint is to adjust the strike on the trunk latch so that the lid sits much lower providing more gasket compression. This makes the lid very hard to close because of the excessive gasket compression plus the lid is no longer level with the body on either side of the lid (big time misalignment). Not a satisfactory solution. I am thinking that the solution might be to try fill in the depression at the lap joint with a little clear seam sealant to create a level surface for the gasket to seal against.