[Sergio]

You are lucky, that blue wire is the positive wire but since you connected it to the negative post there was no voltage.

The negative wire is a 12ga black wire with a yellow bullet connector.

One you get your 48v->12v reducer, what gets connected to that blue wire depends on where you mount the reducer.

If you get a generic reducer and mount it in the battery compartment, then your reducer +12v output goes to that blue wire.

If you get a precedent plug-and-play type reducer that is mounted up front then you would connect that blue wire to +48v for the reducer input.