Battery has juice, But nothing else does. 97 Mitsubishi Montero Sport?

I put in a stereo and when i drive, it would flicker the power to the stereo whenever i would use something else electrical. Everything has a good connection behind the stereo. I drove it this morning and everything was just fine (with the exception of the flicker in the stereo) ,parked it, then 6 hours later went to drive and nothing would work, not even the reading lights. i tried jumping it and still nothing would turn on. Any suggestions?