A6 swimming pool
 

Hi, my A6 has yet another swimming pool in the back passenger floor, I have removed all debris from both drain holes, one under the battery and another further along the bulkhead.
Under a big black plastic electric box, left side of car just before the windscreen, there's water that isn't draining away down to the bulkhead.

Is there another drain hole under this black box?

If so how do I clean it?

Thank you for any help you can give.

Sorry, forgot to say 1999 A6 automatic Turbo estate, right hand drive.