Water leak in 2000 Mazda B4000 truck

Are you sure its an outside water leak? The air has a small tube under the passenger side dash that is not very secured and if it pops off, it will drip on the floor appearing to have a leak.

First you must confirm it is outside water (ie not from cooling system or AC condensate). We had a leak into the passenger side whenever it rained. Large amounts of water getting in through the air intake from the outside and going int the heater. We bi the bullet and removed the entire dash assembly inside, then the heater housing so we could see exactly where the water was coming from. These hours of work were worth it. And the source was, astonishingly, a problem that originated at the Ford plant where the vehicle was built. The heater gets fresh air from the cowling behind the hood. There is an oval shaped dam inside the cowling, just out of sight if you look down from the air intake vents toward the passenger side. The dam prevents water from running down into the oval shaped intake port because its sides are about 1-1.5 inches high. Sounds good, BUT, water can enter the cowling and instead of running down to the channel, around the dame and out the side drains toward the quarter panels, it can wick under the roof of the cowling and drip off the roof directly down into the heater like Tom Cruise rappelling past the security systems. What made this all super intriguing was the following: at the factory someone had run a bead of chalking along the underside of the cowling, maybe 4-5 inches right to the center of the oval air intake dam. And right there added a 1 inch pile of coiled chalking to form a stalactite directly in the center of the oval opening. Being the lowest point, water ran down hill and showered down into the heater. I suspect the tendency for the cowling to wick water and drip it into the heater was known by Ford and the factory was consciously trying to wick it away from the intake dam, but someone miscalculated that day and created a perfect channel into the heater instead. It could also have been malicious. Don't know. Our solution was to remove the stalactite and bead of chalking and add a bead of silicone to guide the wicking water away from above the oval dam. It was an 8 or more hour job and no fun, but the water leak is gone. Before you do the same, I suggest you remove the radio antenna (small job) and use an inspection camera to look from there toward the driver's side while having someone pour water into the cowling and see if you have the same problem.