Glad you pointed that out about modifications. One test is with an Ohm meter, check to see if the fluid in the tank has continuity. As for the harness, if for some reason the harness "wicked fluid" to the controls at the steering wheel, then you have a good argument. I am suspecting something else when mentioning "wetter months" and now you need to pay attention to any interior water leaks onto any controller. One test is with all doors closed, a few lights and have a partner water the exterior (front or rear end) to look for water leaks. I have seen a few with leakage at the sunroof drains into the interior (behind panels / onto control units etc.)
If the battery and or electrical system has no parasitic draw, look at the "removed" modifications and how the vehicle was returned to the stock version.
Sometimes, not all the time, lighting modifications can also cause unwanted / uncontrolled component wake up or inactivity.
Another hint is with the mention "Assy ON" and the wipers operate normally. Check / test for AC ripple and or leaking diodes at the alternator. The same test can be performed and at the fused connections.
You will need an oscilloscope to measure AC ripple.
Been down that road before.
Use key words - Automotive AC ripple test
