Battery voltage drops to 5V with negative connected on a Ford Edge

I disconnected the positive cable and touched the test lamp to the terminal and post, lit up like a Christmas tree. So I started disconnecting things one by one, starting with the alternator. No change, then the PCM and every other harness/electrical connection I could find. No change, the lamp still lights up. I couldn't get to the starter without disconnecting the coolant hoses, so we might just take it somewhere if that's how deep I'm gonna have to get.

Engine running I get 14.2 volts, engine off it's been holding 12.69 volts for 20 minutes. Man, I have no idea...