I am sure John will jump in and clarify this for us,
It looks like what He is saying is " each powered pickup truck requires it's own PTC as close to the positive supply contact point inside of the car as possible. With the powered truck PTC's you would have added protection if there were a direct short between the positive internal truck crossover wires as well as the lighting circuit. "
Thank you for the PTC and TVS - Some Commonly Used Sizes and Part Numbers (from other threads) list rtr12