As cdc
You have a problem, weather they could find it or not. Obviously they not good enough to find the problem, They are one problem, cannot be bothered, some leaks take some finding. Down to them get them to sort it.
IF on the other hand it is out of warranty, best way to find it, is sit in back when it is throwing it down, keep feeling around all the seals, windows doors fuel cap etc etc Not just the back ones, could be from front door posts etc running into back, IF dry after all that, Air-con evaporator will have a drain for water that condenses on the evaporator, some times the drain hole blocks, but that is usually situated under the dash, so water would normally go into front. But check that it is not running into back from front. Do you park the car level, or on an incline, on an incline try parking the other way around.
SD
November 2012