turn on your headlights and turn the key or jump the relay.

if your lights go out you just don't have enough charge.
it could be a bad battery, as 12.5 is a little low to me.
I normally see 13.2 on a good batt.

it could also be the cables, as I have had ground wires go bad and not let me turn it over at all.
in which case you can try jumping the car with pos to pos , ground to block and to neg on the good running car and see if that helps.