Can you give suggestions to find this leak. 99 ranger xtended cab: The truck developed a mildew odor. I found the carpet soaking wet in the area under the seat edges that have like a gully in the front area and the back area of the floor board. I got it dried out, but it has returned. I drive in areas at work and to my house that has puddles after a strong rain storm. I have checked around the door, window , rear side window, and rear window slider window. I have not found any area of leakage.

 

They can be hard to find. My 94 is probably somewhat different than yours, but I had a leak from the rear (slider) window. I couldn't see it until I pulled the interior panels off around the window. It was leaking from what looked like a cut-out in the seal for a mounting screw or bolt, but there wasn't any hardwarethere. A dab of silicone fixed the problem. How I found it was, with the trim off, I took a garden hose to the roof of the cab, and then I could see the water from the inside.

 

I had the same problem with my 91 so I got tired of wet smelly floors so I ripped the carpet out and found two holes via old rubber plugs so I fixed those let it all dry putcarpet back in and it still got wet! as it turned out the flashing around my wind shield was spaced out from the body

 

Could also be a heater core leak

 

Check your cargo lamp. My 96' leaked around the cargo lamp and because you can't just buy a new gasket for it you will have to lay a bead of RTV or auto silicone and seal it back up. If that doesn't work, what I did to find the source of my leak was run the hose over suspected areas one at a timeand waited to see if any of them leaked through. Just some ideas.