My 1977 firebird has electrical issues please help?

my battery has 12.8v but as soon as i turn the key to the on position it goes down to about 1. If i turn on the head lights or hit the breaks or anything that uses electricity it drops to below one volt and nothing actually turns on just seems like I'm opening a short. the only things that are getting power are my starter, alternator, the fuse box. ive tried removing most of the fuses and got no results. my radio is conected straight to the battery, i put it in myself and that works fine but when i turn on anything else it just loses power. ive tried jumping it with the same results nothing different.i think theres a short somewhere but it doesnt seem shorted till i turn the ignition or use headlights or hit the breaks for tail lights (there the only things i can use without the ingnition switch on)