So here's the deal:

Out of the blue, my passenger floor('98 Ram 1500 5.2L) starts flooding with clean water. I have no history of a/c issues, or engine problems in general. This has me perplexed. Anyone else had this issue? Please help.

 

I have a 1997 ram and I can tell you exactly what it is. Mine started doing the same thing last year. I surfed the web and found all sorts of quick checks and fixes that did not pan out. I finally ended up taking the entire dash out to get the heater unit out only to find that it was perfectly fine. big headache. I finally solved the mystery in that the condensate drain where it goes through the firewall was draining back into the cab. After a few years the insulation or sealer to keep it out of the cab begins to deteriorate. The fix is to get a piece of tubing and a 90 degree elbow from lowes, home depot, etc that will fit over the condensate drain on the engine side of the firewall so that the water drains down instead of back into cab. Fit the tubing over the condensate drain and put on the 90 degree elbow facing down, clamps on and your done.

 

he nailed it, same on dakotas. The condensation runs out the tube and runs right back in cab. My old trucks floorboard had so much water in it it would puddle under your feet.

 

Agreed, blow out that drain tube with compressed air, get a 90 degree turn down, and reseal the tar like substance that encapsulates the drain tube exiting the firewall.