freeze plug?

What is your question?

Car has a water leak at the back, underside, by the firewall, I think. I cant see it. I stopped driving it when it started leaking. I have decided to fix it. I noticed that the heater did not get warm this winter when I did start it up. One person said it was a freeze plug that went out, but Ive wondered if it was something else since the heater stopped working. any thoughts?