Right now, the only voltage you have to the trailer is 6V being supplied by the small gauge red wire with the yellow crimp on connector on the positive terminal, and the white wire connected to the negative terminal. The battery on the right side is not supplying anything since there is no connection to the positive terminal. This rig is miswired and you need to trace all wires and determine the correct positive and negative wires. Be careful when you hook up, or you could blow the converter fuses if you wire it incorrectly.