Probably just the water leaking in the IPDM is shorting something out.

 

I took that apart yesterday and it appeared to be fine, the IPDM itself was dry and there was no corrosion. I dusted the IPDM because it was a tad dusty, but I have seen dustier boards in computers that run with no issue. I sealed the box up tight so no water should be getting into it unless there is standing water. When I took the IPDM out of the battery compartment where it sits, it had about 1/4 inch of water because the drain plug was plugged up. I cleaned it out and dried the compartment up, so there would be no more standing water. I don't know that it would be the issue after doing all of that. I will probably go pick another IPDM up on the weekend from a local salvage yard just to try it though.

 

Did you disassemble all the way down to the board components? There are some soldered relays that may have condensation inside them. Bag of rice trick like for a cell phone might work.

 

This^^^, and also did you disconnect the wiring harnesses and look inside the connectors?