Whilst I was changing the fuel filter and pump in the rear wheel arch, I had a look beneath the plastic sill end cap to see how things were bearing up. Just a couple of screws to remove it and mine was full of damp sand which must have been there over a year! I'll take the other side off tomorrow and empty that out too. I also cleaned off the rust and slapped some paint on, but I suggest you spent 5 minutes and check your end caps too before salty water does any harm!

drill a hole in it, rain will keep it clean

picture is from front but made the same on back

thanks for the heads up.
I had reason to remove the rear mud flaps for some work.
I could not believe for muck it was hiding.
have a good weekend

Thanks.... will take a look.