Z cars have this mysterious ability to end up with water on the floor under the mats after a hard rain. Both my '70 and '76 had this problem and I could never track down the exact source. One day I decided to remove the fan under the dash to repaint the firewall. Guess what I found? It wasn't leaking from the windshield corner, it was coming from the body seam between the firewall and cowl. Late 1960s Ford Mustangs had this problem too.
ONE CAUSE: Under the windshield-wiper cowl there are two very tiny drain holes under the crossbraces on each side which I never noticed before. Not the down spouts under the fenders, these drains are under the wiper motor in front of the windshield. Well, rain water flows under these from the center out to the large drain tubes on the side of the fenders. These small holes get blocked with old dirt and let water pool around the center wiper motor area(!)
If you're drilling holes through top and bottom plates or running wires through a fire wall in the garage, you must seal those holes with a fire-resistant caulk or foam sealant to comply with fire and energy codes. Most building officials won't make you bust out large holes in ceilings and walls in order to access hard-to-reach holes, but check with your local official before you begin your project. Fire-blocking insulated foam sealant is sold at home centers and hardware stores. For more on working with foam sealant, check out this collection of helpful tips.