If you have a leak where the glass is inside the rubber sheath around the pillar, take 3M black weatherstrip and place the tube's tip in between the glass and rubber. Squeeze the tube and over fill, so a bead will be squeezed out as you work your way along the rubber. Whatever squishes out, clean quickly with small artist painter's trowel. What small film remains, use a cloth and brake clean to remove the rest. Use the brake clean sparingly.
Want to pinpoint were the leak starts? Take a garden hose and have someone sit inside the car. Run the hose without any fittings on the end, and turn the water pressure on lightly. Start from the bottom of the windshield and work your way up. When the eyes in the car visually sees where water is coming in, you have your starting place. As you travel higher along the windshield's rubber with the hose, they might see just how much or little the length of the leak is.
Thanks for the great tip on diagnosing the location of the leak and the fix. Did you see my post asking about a source in or around Santa Monice for rear window Plexiglass for a 59? Or better yet do you have any tips about where to get one?
You are quite welcome Mike for the trick tips. Thanks for the kind words. Hope it works for you.
I do not know any source for your glass... sorry. But what pops in my head, are all those drag racing glass cars that need fake windows in them. I would try searching for racing shops that specialize in plexiglass frabrication. Maybe they can direct you to someone in the industry who has just what you need. Good luck and Mahalo.
Thanks for the tip. I just returned from vacation in Baja. While I was traveling through LA and looking for a local supplier I got the name of glass works as one of the leads. It is nice to get a second reccomendation to cut down on the trial and ERROR factor.