Heter matrix is located in the middle of the dash board, open the bonnet and look at where the two coolant pipes go through the bulk head. Stick you head into the passanger footwell and look up and under the center of the dash, and you will see the pipes going into the vack of the matrix, there is a little plastic rectangular cover that sits over the end of the matrix, pull this off and if it is wet here, time for a new matrix, cheep and not difficult to change.
If it is dry, have a look at the gromets where the wiring comes through the bulk head, if not maybe the windscreen seal/metal work, but the sunroof drain channels can get blocked/fall off, so they are worth a check too.
Hope this helps
cheers
kraM