I had a leak on the passenger side like that.
It came from two places. One was where the A/C unit met the kick panel sheet metal. It was a poor fit and water would come down the inlet baffle, behind the fender, and drip through the poor fit. Black silicone fixed that. The other, which sounds like yours, was the windshield seal. Where the factory cab seam is there was a poor seal to the windshield gasket. I had the windshield removed and resealed using the same gasket and the proper sealant which, I was told, is not RTV silicone.
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'93 W350 Club Cab, 5 sp. , Factory Options: LE, 4. 10, LS, 7600 lb. rear springs, tachometer, front stabilizer bar. Mods: Banks stinger plus, Linex bedliner, 25,000 lb gooseneck, 10,000 lb receiver, Tekonsha brake controller.
'Power Wagon' injectors to be installed!