Mine had that problem, it would only show up after rain and the blocked sunroof drain caused the passenger side carpet to get wet.
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I have now linked my GAP tool to the car and had a bit of a look at what I can do with it - lots to learn there!
I accessed the left hand wiring loom to check the 3-wire splice, which looks ok. While getting access I found that water had leaked in, via the A pillar I suspect. When I removed the A-pillar finisher I found a redundant hole that should have been sealed over was open. Both sides were the same with much more water on the left side. I'll be removing the front seats so that I can lift the carpets to dry everything out.
Hoping to check out the defect messages tomorrow in more detail, in general terms there are a number of communication failure messages. Could that mean CANBUS problems?
This how to could come in handy for you
FAULTS EVERYWHERE AND A WET FLOOR, IS THIS THE END FOR DISCO STU! - YouTube
Water on the passenger side floor will generate suspension and transmission faults. After you dry the floor you may want to clear the faults and see what remains. Also check for corrosion at the plug connector for the Transmission ECU behind the battery.
Normally you can ignore the comms faults (U code) to start and focus on the others first - starting with any P coded Powertrain codes - but in your case take note of all the fault codes first (I just take screenshots).
Today I replaced the 3-wire splice and mopped up about 2 litres of water that had entered via the A pillars. All of the previous faults were still displaying on the gap tool and the instrument panel was lit up like a christmas tree. The last time that the car worked properly was just before the body was lifted to work on the engine, so I thought that I'd check the connectors that were disturbed for that job.
Starting at the rear left wheel arch I cleaned and reseated the group of 3 connectors there. I put power back on and all but one aircon related message have cleared. I still can't start the engine and the remote doesn't lock/unlock but all of the other systems seem ok and there are no fault messages on the gap tool. When I turn the key to the start position, I hear a relay click in the junction box but the engine doesn't crank. The only lights on the dash are oil pressure, generator, seat belt and park brake. The park brake now operates.
I'll clean some more connectors and study the Gap manual a bit more!
Progress there Dave.
Today I refitted the original main junction box (MJB) after cleaning the connectors. Put the battery in and found that now the remote works, I can lock/unlock the car. With the key in the ignition all seems as I would expect except that the engine will not crank or start. Faults displayed on the message centre were - bonnet open (it was), low washer fluid, park brake fault. I will connect the gap tool and see what that tells me.
I'm thinking that I may need to reload the key as I had tried to load the key when I had another MJB installed.
The FOB working is a good sign, maybe try a software reset thru the GAP tool.
I put the gap tool in today. The main fault codes seem to be B1C57-14 (2F), B1D12-15 (2F), B1B02-28 (2F) in the body control module. C1A48-01 (EB)park brake module. B1B78-84 (2F)ventilation module. The engine still doesn't crank and I have reset the key via the Gap tool.