How would rain water accumulate on the drivers side floorboard of my 2003 Dodge Dakota?

Most likely through the door weather strip. Less likely through the windshield weather seal. You can buy a kit at any auto parts store that consists of different colored dye tabs, place them around the windows and doors and run some water over them with a hose. Depending on which color leaks through will tell you where the water is getting in. There are some other potential leak sources that are harder to detect, namely the plugs in the floor pan, if one is missing water can splash up and soak the underside of the carpet or foam padding. Your heater box drain can also foul up and condensation from the AC/heater core can eventually spill over into passenger compartment.