I agree with the curtain wire method, after seeing posts before and having my old Ls400 suffer a leaky rear end.
Couple of quid from local hardware dealer and a bit of fiddling, wasn't the fix for me as the drains were clear.
The sunroof rear drain holes are the hardest to get to from the top as they are at the back corners of the sunroof and you have to feel your way with the wire, the fronts are easy enough, they are in the front corners.
Word of caution, don't go like a bull at a gate, easy prodding to save on causing damage.
As to where they come out under the car i cannot say.
Is water in the center part where the spare wheel sits ?
Not sure what to say as to where the water is getting in, could be as a result of the blocked tubes or leaking through bottom of screen area, just have a good look about for water trails, they usually leave a sign.
"suffer a leaky rear end" Can there be a more antisocial problem ??????? Mike