The junction box is tiny. Did you install it yourself? Are you familiar with what is inside the frame? Do you understand that it plugs together and the junctions are covered with heat shrink tubing? Typically the junction box is stuffed in the downtube. If you have a BB30 or PF30 BB the wires are usually run underneath the BB shell. It's a tight fit, so you may need to poke and prod them a bit to get them back through the hole in the BB shell that's covered with a plug. I suggest that you pull the crank first. Chase the junction box back through the BB hole, then find the RD cable. Slit the heat shrink carefully with a razor. Disconnect and remove the old cable. Fish the new one in from the RD end of the chainstay. Put a new piece of heat shrink over the cable end, plug it into the junction box, and seal.

That RD cable is about $9.