95 cutlass supreme leaking coolant from what looks like firewall overflow plus steams upwindshield

Your heater core has gone bad and needs to be replaced, If you go to Auto Zone you can pick up a new heater core and they will have the step by step instructions to doing the job yourself. If your lucky, may be the radiator hose to the heater core has failed at the cores pipes and it's only the hose that needs to be replaced but thinking not. You can get by pass the heater core for now so you can still get around till you pick up the new Heater core. Follow the two hose that goes to the fire wall and disconnect both of them from the pipe from the firewall, then with a pipe and two clamps, connect the two hose together and that's how you by pass the heater core. Good luck friend and hope this helps.