here is exactly what i did:
had this exact same problem on a 62 plate EX First i had engine management light and connected the computer gave code P0504 (BRAKE PEDAL SWITCH) then shortly after ABS Light, Electric Steering Wheel and Flashing Tyre Pressure Horseshoe.
Here is how i fixed it:
First press the Brake Pedal a few times and if the brake lights work its not the Brake Switch don't even waste your time taking it out, its just a switch there is no circuitry in there (and Honda charge near £60 for a new one!!)
The next thing i did was check all the fuses with a multi meter but if everything is working in the car i would skip this.
Just go straight to dropping the back seats and get in the back look at the ceiling either side by the boot at the top by the ceiling airbags if its wet or water stained that is the issue.
remove the side plastics start with the drivers side cos this is the problem area pull the rubbers and get the plastics off be careful with the top grey plastic it pulls downwards but the airbag is above it, just rock it back and forth and it will come off .
you are then greeted with 4 or 5 white plastic wiring plugs all soaking wet!! i disconnected them all blew all the water out sprayed WD40 inside them then dried with hairdryer. I was thinking about removing the plugs and soldering each wire and insulating but its a bit of work too do that.
anyway once its all dry plug it all back together then i got some plastic bags like food bags wrapped around the wires at the top of each plug and secured with a cable tie then duck tapped over it and left the bag open at the bottom about an inch past the plug.
so basically waterproof each plug but leave the bottom open so it doesn't condensate, do all the plugs even the yellow airbag ones just pop the clip off the body, i didn't clip them back just cable tied together as the water comes down the bodywork.
do the passenger side too not much over there just some DENSO stuff and the airbag but cover them too.
Now i still haven't managed too find out where the water is coming in but i've just bought some STICK LIKE SH1T CLEAR from Screwfix so i'm gonna seal the fook out of everything around that area and hopefully it will keep the water out.
put it all back together turn the engine on and tap the brake a few times and all the lights should disappear.
happy days
mine started with the alarm going off random at first so that must of been the start of the water getting in those plugs just glad its not Gremlins