YES, leaking Mustangs that is a common word with Stang owners. My son’s 67 leaked from the Cowls and the holes were past the metal dam so the plastic inserts didn’t help. We had to repair the cowl panel; I just cut the cap covering the Cowl dam under the fender which I found a beach from sandblasting from the previous owner. I cleaned and applied Eastwood’s rust inhibitor paint then seamed sealed the area around the cowl dam. Since I cut using a body saw which leaves a thin gap I was able to stitch weld the panel back to where you can’t tell they were off. If the rust damage is bad where the holes are large then the cowl vent must be replaced. I see there is now complete cowl vent grills available for the 67-68 Stangs and soon for the 64-66. Make sure it is the cowls before tearing in to them because the windshield seal can leak. If you want pictures I have them in the scrapbook of the project.