You'll end up having to take the headlining out imho.
The problem is MOST likely due to the fact that the sealant between the sunroof plastic frame on the outside of the car and the bodywork goes hard and peels off. Mine had the same symptoms, leaking from the electric motor housing (mine's automatic, obviously...)
The other thing to check while you are there is the drain tubes and the plastic boxes they attach to as the sealant joining the boxes to the metal frame tends to go. Search for a thread describing using flashband bitumen tape to reseal it.
David