Chevrolet Repair: under dash interior leak when it rains, rust holes, gm cars

Question
Thanks for your help, I have several gm cars 1977 - 1983, 3 of which leak on the passenger side front floorpan when it rains. I replaced the windshield ribbon on mom's 83 buick and also took apart the heater core box to look for problems. I did notice the entire plastic box is attached to the firewall with some sort of super gm glue. I didn't take it all the way off but did my best job of putting silicone around the sealing areas the best i could. I did see the drain hole in the box and it was clean. All of this and it still leaks although not as bad. My 1979 bonneville has a pond in it today after 1" of rain. I haven't looked in to this yet but it seems that all my gm cars of this vintage must have had problems with leaks as the floorpans on the front pass. side are either rusty or completely gone. Any ideas would help ease my frustrated mind.. Thanks, Duane in northern Arkansas

Answer
HI DUANE, WELL I HAVE SEEN THIS MANY TIMES AND ABOUT 80% OF THE TIME IT IS FROM THE FRONT WINDOW LEAKING..WHEN YOU REPLACED THIS SEAL AROUND THE WINDOW DID YOU REMOVE THE WINDOW AND PUT THE SUPER SEALER AROUND THE METAL PART WHERE THE WINDOW GOES TO..THIS IS IMPORTANT BECAUSE WHAT WE HAVE FOUND IS RUST UNDER THE WINDOW AFTER THE SEAL WAS REMOVED..SO WE CLEAN ALL THIS OUT AND PUT A RUST SEAL LIKE PAINT THAT STOPS THE RUST THEN PUT DOWN THE SEAL AND THEN THE WINDOW AND THAT USUEALLY TAKES CARE OF THE PROBLEM..WE HAVE ALSO FOUND THAT IF YOU REMOVE THE COWL PANEL UNDER THE WIPER ARMS YOU WILL SOMETIMES FIND RUST [HOLES] UNDER THE PANEL AND WATER WILL RUN INTO THE FIREWALL FROM HERE..BUT YOU HAVE TO REMOVE THE COWL PANEL TO CK THIS OUT FULLY.. THIS IS ABOUT THE ONLY PLACES THAT WE SEE THAT HAVE A PROBLEM..GOOD LUCK TO YOU AND I HOPE THIS HELPS YOU,,,TOM