Getting + Voltage On The Neg. Terminal

[69 Short Fleet]

I got my battery charged up and its at 13.2 volts. I hooked up the pos. side only and checked for voltage between the neg. battery terminal and the end of the neg. battery cable and I'm getting 11.2 volts right away and then it quickly drops to about 5.8 volts and holds there. I pulled all the fuses (one at a time) and kept checking and it is still the same? I tried disconnecting everything I could think of horn relay, horn, emergency flasher, signal flasher and still the same. Can someone steer me in another direction, I don't get this at all..... Thanks Doug (key is in off position and everything is shut off)