Summarize:

Heat guns can bring out the natural oils in black plastic and restore its luster, but this is not a long-lasting solution. The plastic may eventually fade with use and, after several treatments, will no longer have enough natural oils to draw out through heating. How long this restoration method lasts depends on how often your car is exposed to direct sunlight. The more often you use you car, the quicker the restoration will fade. If you've used a heat gun in the past but the treatment no longer works, try adding olive oil to the plastic's surface to restore its shine. You can buy or rent a heat gun online or from some hardware stores. Heat guns can warp or discolor the surfaces of non-plastic items. If your object is attached to something, cover any areas you do not want to heat with a flame-resistant tarp. This method is primarily for car trims and bumpers. Do not use this method on black plastic attached to flammable materials (like some plastic toys). Using a heat gun on dirty plastic can burn in any dirt or stains. Wash the object with soap and water, cleaning away as much debris as possible, and dry it off with a towel before applying heat. Turn on the heat gun and move it in small circles around the discolored plastic. Avoid leaving the heat gun in one place for too long to keep the color treatment even and avoid burning the plastic. Test the heat gun on an inconspicuous area first to make sure you like the treated plastic's color. As you move the heat gun around the plastic, it should change to a darker, richer color. When you've covered the entire surface, turn the heat gun off and check the plastic. If you're happy with the new color, you have finished restoring the object. If your plastic still looks faded or discolored, try applying olive oil or painting the object instead.
Use a heat gun as a temporary solution. Cover nearby non-plastic objects with a tarp before using the heat gun. Clean and dry the plastic object. Hold the heat gun several inches away from the surface. Tun off the heat gun and inspect the plastic's new color.