95 Riv Supercharged Leaking Coolant Inside Car

95 Supercharged Riviera. On road trip from CA to Phoenix, coolant started leaking into cabin of car, about 1 inch deep. Got oil changed in Phoenix. Came back to CA, few weeks later replaced water pump. Car has had almost no maintenance. Check engine light is always on. Car gets zero gas mileage, and goes through coolant quickly. What needs to be done besides an oil change and a tune-up? Will the radiator need to be replaced? Is there a leaking hose I should look for? Thanks!:banghead:

Likely the heater core.