[Grumpy old man]

Have you checked the drain behind the vent ? It Looks like your windshield gasket is leaking , use flow grade windshield sealant on both outside edges of the gasket , lifting them with a non marrying glass tool or just a popsicle stick with the sharp edges sanded off to lift the gasket edges enough to use the sealant ,of course you would want to protect your new paint using low tack masking tape .it's probably coming in from the top of the windshield at the body seam and flowing in . An easy trick to find the leak is to cut strips of white paper and insert them from the inside of the truck into the inner gasket edge then run a hose over the truck the paper will reveal the leak .