Changing truck cargo light into third brake light+cargo light?

Well i have a 1990 GMC Sierra that only comes with a cargo light where most third brake lights are... I really want to get a third brake light and cargo light to replace the current cargo light... What kinds of things would need to be done to accomplish this? Ive noticed that the new light only seems to have clips that plug into the old plug but i think its only for the cargo light portion...|||You%26#039;re going to have to Rewire the light; Wire to a Socket with a Two (1157) bulb so one is the Brake %26amp; the Other Element the Cargo Light but it will last longer if you go with 2 Lights, Stop %26amp; Cargo. (Both Bulbs 1156)