take some paper and line the area below the windshield, spary it with a hose or wait for a good rain storm and you can see where if any water is coming through.
I'd be more inclined to suspect the fresh air intake, that grill between your windshield and hood is where the air comes in for your heater, there should be a drain for removing water that may be plugged or the ducting may be cracked.
"But it does me no injury for my neighbour to say there are twenty gods, or no god. It neither picks my pocket nor breaks my leg." - Thomas Jefferson, Notes on the State of Virginia, 1782
Arguing with a truck driver is like wrestling with a pig in mud, eventually you realize the pig enjoys it.