I am trying to upgrade our thermostat to a Wifi unit that needs a C-wire. Found the unused C wire at the thermostat end and connected it. AC wouldn't power on, went to furnace end and found it wasn't connected there either. I am not sure which of the nuts to tap into to power the C-wire and would appreciate any help. It is a Goodman unit, I found some previous posts with the same unit but the wiring on those didn't seem to match mine.
EDIT #1: Added more photos. Looked all over panels (including the inside cover) and could not find a wiring diagram. I can confirm it is a Goodman GMP050-3.
Thanks all, especially Dr. Sparks.