2000 Monte Carlo SS Electrical,,freaking out

On my 2000 Monte SS I got in to start it the other day, and when I turned the key all kinds of lights flashing, and nothing on the start. I hear buzzing coming from the radio and relays clicking and all the dash lights and radio flash. Its posessed. When I turn the key to run all goes out and nothing. Key to acc, and all the flashing goes on. I cleaned the positive cable on the battery but still the same. Any thoughts to where I can look from here. In 30+ years of working on cars , ive never seen this happen, ever. I am at a loss. Thanks for any help.