First things first: you must know how to solder in order to make this possible.
1. Take your battery out!
2. Now check the back of the RBX and take off the big black plate. Yo will see a lot of wires.Check, if there are any loose wires with no contact to anything. If there are such wires, you've got a much bigger problem.
3. If this is not the case, search for the wires that are coming from the battery (red and black) and follow the black one.Now you are on your output jack. There are two other wires connected to it. A white and a thicker black one. The black one is ground. It has to go on something metallic, like a potentiometer. Yamaha has made the mistake to create a short there because the output jack is also an on-switch.
Now for the important information.
The output has 3 contacts: tip, ring and ground. If you put a jack in it, the circuit is completed and the battery is "on".
4. To achieve this, the wires have to be connected to:Battery negative: ring, the shortest one of the contacts.
Ground: ground. Obviously. The longest/biggest one.
White/positive: tip, the contact, that is left if you followed the order. (Or the longer of the short contacts?)
It worked for me but I take no responsibility if you make a mistake and destroy your bass;)