The easiest thing as other have mentioned is just to swap the plugs to the motors on the RX 18. Simply by unplugging them and swapping left and right might fix the problem. If the gear set you put in has been stepped down to 3:1 the extra gear means the shafts will spin in the opposite direction of the regular gear box. If that is the case you either have to swap the leads on the motors or the jacks

Cheers