Coolant Leak?

I would look over the heads as a leak on the rear of the block would mean the block is cracked. There are no freeze plugs there only oil passages and those are screw in plugs.
However the heads do have plugs in them that could rust through. It's been a while since I've touched one but you should be able to use a mirror to see the plugs to see if they are the leak. If they are not then look at the firewall to see if it's a leak in a coolant hose. If not there and it's still in that area then you will need to get under the car and look through the inspection port or the starter portal. Hope it's just a freeze plug or hose. A cracked block is no fun.