Unless the windshield is obviously cracked, there are only two other possible causes to a leaky windshield. One is from the seal around the whole glass, because it's broken or worn by age, and the other is the more common culprit on LLV postal trucks, and that's where the side mirror bolts on. This is is hard to see without a mirror and flash light, but check it before you replace the windshield, because if it's the source of the leak, you'll still have a leak even after replacing the windshield.
The United States Postal Service (USPS) uses these "Long Life Vehicles" for most of their mail deliveries, and if you're the mechanic on the job and you've got some more problems, check out the other LLV repair videos on WonderHowTo.
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