i wouldn't mind knowing this too, i think, you can get an air compressor and blow it down there.
OR
there was a Shitreon design fault, where somewhere in the passenger footwell, there is a bolt ( long bolt ) that clamps the pipe, so therefore you need to unscrew it a bit to release the pipe, but if it's both the pipes blocked then it's probs best to just blow down it.
how did you know it was block? i took mine off and there was no water? dry as a bone:/ perhaps the actual gutter in the sunroof is blocked!