When you wet the windshield, does water get to the area behind and aft of the brake booster and fuse box?

Read through this - PeachPartsWiki: Blower Motor Removal - then remove the black plastic trim ahead of the driver. You might have to set the wiper to vertical but IIRC no need to remove the wiper. You,ll see the base of the windshield and you might get a better view of the leak source.

Sixto
87 300D