Hi Geoirge,
Using the messenger that you have already fed through, attach a bundle of rag to the messenger at one end of the boom and try pulling that gently through. Hopefully it will snag against the car and pull it to the end of the boom. Take care when the car arrives at the aft end of the boom to ensure that the four little nylon wheels fitted to the car, but which are not actually secured to the car, do not drop out and bounce over side. They are not desperately expensive, but I bet you haven't got any spares onboard to replace the ones you might loose. If the rag does not snag against the car, you could fashion a small hook out of an old wire coat hanger, and attach it to a long thin bamboo cane or thin metal rod which can then be poked down the length of the boom and turned to engage with the car. Good luck !!
