Arrange things so you have easy access to the underside of the circuit board, and the back panel to the right.

The contact points we need to work on are the bottom 4 pins in the cluster of 6 pins, directly left of the switch, on the back of the circuit board.

For the Powered speaker on the RIGHT, connect the top and bottom connections in that group of 4. Two parallel lines. ||

For the Powered speaker on the LEFT, cross the top and bottom connections in that group of 4. Two intersecting lines. X