By any chance are you in North Carolina?
I ask because I had this very same problem happen today. Or, rather it was noticed today.
It started when I drove the car this afternoon. The rear wiper started coming on sporadically, then the problem proceeded to the turn signals not working. I also had the issue with the parking lights not turning off when the car was parked and the light switch turned off.
Luckily, I made it home and my husband discovered the exact same problem, that you are describing with the front passenger floorboard being wet and the rear passenger side actually holding water in a compartment under the seat.
This is also the area where the main essential fuses are located. He unhooked the battery in order to get the lights off and we have been trying to figure out what happened to cause all this water to be trapped in the car.
Our Mercedes is a E-320 and we have had it for three years, but this is the first time this has happened.
Was there permanent damage to the fuse box in your Uncle's car or what was the repair needed. I know each incident is different, but I am cringing at the thought of what will have to be replaced.
Good luck and I hope others will chime in with their experience with this issue.