I haven't come across any drains under the dash, but my 05 Roadster has four drain stubs emerging from the heater box. I've recently fitted some clear pvc drain tubes to each of these so that I can keep an eye on what drains out.
DaveW '05 Red Roadster S1 '16 Yellow (Not the only) Narrow AR GDI Plus 4
I've recently fitted some clear pvc drain tubes to each of these so that I can keep an eye on what drains out. 
