This can be done in either Jimmy's Vendetta or The Betrayal of Jimmy. To keep the target car in a Car Dealership mission, first start the mission with the desired vehicle. Once you get inside the vehicle, get back out, and get into another car (preferably the one you started the mission with). Get back out, and into the target vehicle once more. At this point, finish the mission, and at the scoring screen, choose to REPLAY the mission (not continue). The replay should begin with the target car as your vehicle. Drive it to a garage, store it, and then finish the mission again. Now you have the vehicle in your garage, and can use it as you like! This can allow you to obtain cars that are impossible to find elsewhere, such as the Culver Empire in the Betrayal of Jimmy, and several 1940's-only cars in Jimmy's Vendetta, as well as the Prison Bus (although the bus can't be stored in a garage).