98 Grand am GT Problems (PLEASE HELP)

hello everyone, ok here's the problems, and before you even think it, I have and have used the Haynes repair manual to no avail.. First my car wouldn't shift out of park or any gear without excessive trying, and then my heater core went out.. I got a new shifter w electric actuator and heater core.. Installed shifter but not heater core cause I couldn't tell which black housing heater core is located. Started up my car and after about a minute, car went completely dead. checked battery and it had power. Now exterior lights, alarm chime, interior dome lights, turn signals all work.. Radio, AC, PWR Windows dont work, when I turn the key, nothing happens, sounds just like trying to start a dead car..
Please someone give me some ideas before I have o spend a SH&^Load of money for repair