97 Buick Regal - Fill Coolant Light came on, the tank was drained to the bottom. When I fill up the radiator reserve tank the fluid is coming out of a small, 1/2 inch diameter maybe, black rubber hose sticking out a little from the firewall. I can't tell if something fell off or not or if this is some overflow line. As I fill it the tank drains to the bottom out this hose.

The heater core inside the passenger compartment is leaking. The water is draining through the Air Conditioner condensor drain tube. Don't plug the drain tube, the passenger side floor will get wet.

First, drain cooling system (or be quick) and disconnect the two heater hoses that are going into the firewall and splice the ends together with a 5/8" garden hose splicer.
The leak will stop, but your heater will not heat the interior.

Replace the heater core....