You didn't mention what year your Jeep is, but at least on a 97-03, If you simply wanted a door switch on the tailgate to operate a cargo light, you could splice into the Pink/White wire in the sound bar (hot) and run it to your light. The second wire from your light would run through the switch, then ground to the body.
If you want the dome and the courtesy to come on with the cargo also (when you open the tailgate), do as above, but also splice in to the yellow wire in the sound bar and run it to the side of your new door switch that also has the lead from your new light.
If you want it to act like the courtesy and dome light and come on with the headlight switch as well as any door switches, then it's a different story. You would have to run three wires back from locations under the dash and would need the fsm to locate them.