1974 450sl starting problems good battery

[Ricardito]

I had a problem with my seatbelts when my child's seat was in the car. Disconneced wires. Worked good. Installed new lights in the instrument pannel. After putting the airconditioning knob in and turning it, it has refused to start. No lights, no power to the fuse panel. The only place I get power is to the buzzer when I put the key in. Any ideas? 1974 450sl