There was a thread started by Ben I believe regarding this issue. Leaves and gunk clogging the drains. Something about sloshing H2O inside the firewall. A potential real rust nightmare. There were pics in the thread I believe and good advice about what to do, where the holes are. I remember being able to find them and not discover any trouble. Just old dust or sand being the history of the car was CA and the Virgin Islands.
For some reason I think that another thread contained pics of the firewall, dash sliced in half lengthwise... maybe it was Walter's 220SLD donor car -- can't remember if those pics were helpful, or just interesting. Anyway, the thread by Ben I think was exactly helpful to me. Search.
1967 230SL (Manual, rustless driver)