[Sergio]

You should have the +12v going to one side of the relay.
The other side of the relay should go to your fuse box.
Both blue and brown wires should go to the fuse box.

Make sure the original Blue spade connector coming from the +48v pack has been disconnected and taped.

Make sure the accessory Negative coming from the pack is connected to the pack -48v.

If that is what You did, it should work.