Water dripping through firewall behind the dash

Water through firewall - I’ve got rainwater coming through the firewall of my 1973 Plymouth Scamp. For disclosure, am investigating tomorrow but wondering what might be the cause of it. When driving in heavy rain it’s actually dripping through the firewall under the steering column, behind the dash (no doubt past/over electrics) and into footwell near pedals. Any ideas on where it might be coming from and how to fix?