Welcome to the leaks, and good luck finding the forum.
On a serious note, if it's not the drains then it could be the brake light in the boot leaking as the seal perishes or the unit disintegrates. Or as mentioned the screws on the tail lamps can leak. Or another common one is the aerial grommet. Some folk have had the boot hinges leak where they bolt to the apature. The water in the cab wouldn't be connected to a leak at rear tho. That may be another issue unless it is just condensation.