Ford Crown Victoria police interceptor

I have a 1996 that has what appears to be come electrical problem. It started last august that when the air conditioner was plugged it and you turned the car off, when you went to restart it eerything just went click and shut down the power in the car. If you let it sit again it would let you turn the car on again, but as soon as you tried to turn it to crank it would go click and no power anywhere. So we unplugged the air conditioner and all was well until last week. Now it does that power click off thing any time you try to start it. The alternator, starter and battery are all fine and have been checked. Everything is good to start but when you turn it to crank it you hear a little click and then nothing has power at all not even the doors and inner lights. What gives?