ive had this problem and have had to remove the passenger flooring twice to dry it out.
So basically i have ran a load of mastic along the bottom and underneath the bulk head at the back when this dried i then put 2-3 coats of protectakote over it about 4 inches high onto the bulkhead and the same on the floor, for the big gaps at either side of the bulkhead i just used some expanding foam to fill them in let it dry and then cut off the excess and then put 2-3 coats of protectakote over these as well since having done that ive had no issues. You may find water will also get into the footwell next to the sliding door at the back and leak into the passanger side footwell i also took this off and used expanding foam to fill the gaps with some protectakote and finally i put the plastic that fits into the footwell at the back sealed with mastic underneath on the edges and used a few self tappers to hold it down tightly on the edges and 2-3 coats of protectakote around that too this has solved the problem for me.