You could just buy a slew of batteries, and then when one goes dead, mount the next one,,, you'd probably get a good price as well, maybe like wholesale, so you could help the rest of us out when we need one,,,,,, and then you would have a continued use for your trailer.
I really don't know where they put those wires, but if you look in the lower faring on the right hand side, just behind the water bottle, you might come up lucky.