If you pull off your little brake light access panel in the cargo area (driver's side specifically), and locate the wires going to the cargo light, you can splice into those and that way whatever you're powering only gets power when the map lights/cargo light are on.

The easiest way to locate the wires is to stick your hand in the access panel and push the cargo light cover out (towards you, from the back) and then follow the wires back once you can see them.

P.S. Make sure you let the light cool down a bit because it WILL be hot. (Personal experience )

-Eric