it appears everything is in the base to run off of accessory power. The trigger is when the common is completed in the circuit. perhaps you can make a contact using the throw bar to touch the outside rail in the out position instead of an isolated rail being completed by the wheels.
maybe on the motor side, between the motor and the outside of the rail, on the throwbar. A little drop of hotglue and it will touch the wire to the rail when it moves to out.