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If it's leaking in the top two corners of the windshield, it may not be the actual windshield or butyl seal that's leaking.
There is a seam at the top corner where the "A" pillar and the roof meets. If the caulking gets bad in this seam, it will cause water to leak through the seam and around the windshield.
You can buy 3M's Auto Body Caulking and sealant from most any auto paint store, and this will fix the problem with the seams.
If the caulking is cracked, it would be a good idea to dig the old caulking out of the seam and redo it. Also check the "B" pillar seams, as this also can cause leaks at the back, and rot out the lower cab corners.
Don't use silicone, it is corosive to the sheet metal.
Never use expando foam to stop leaks, I did and that caused MAJOR rusting up along the driver side A piller/windshield area. For tempory fix, I used that foil AC duct tape and taped the top of the windshield. That tape holds up for a long while! So far 3 years on my Bronco2!