Leaking when it rains? Where in the hell is this water coming from?

I can almost guarantee 100% that the bellflange where the heater bos bolts up to is rusted to some degree! Under the fender, right at the top where the cowl meets the firewall there is a small opening where the water that goes down the vent slats is supposed to drain out! One leaf or some debris clogging up the hole is all it takes to puddle the water around the bellflange and start it rusting! Fast forward many years and I think you'll find that area has at least some pin hole rust, and possibly worse! A VERY common problem with the early A bodies, almost every early Cuda I've taken apart has this problem on both sides!! There is no easy solution to fix it either, although some Solar Seal or really good solvent based caulk will slow down pinhole leaks for a while!! The only permanent solution is to cut apart the cowl and replace the inner piece!! Look for PabstXX's restoration thread, where he chronicles the procedure!!!