leak either from the windshield of front vents above the firewall. water leaks onto the floorboard under the carpet.
Possibly a plugged drain for the AC spilling into the truck. Check that first. If that doesn’t help, it may be the windsheild seal. A hose and a helper can help you find that.
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If that doesn’t help inspect in front of windshield outside for damaged or clutter in vents that let fresh air into vehicle.
Not sure about GMC’s , but common on Fords
Did this leak start after replacing the bulbs in your center high mount brake light / bed cargo light ?
I know, you think I’m nuts, but the water can make it to the front.