Wiring for replacement cargo lights:
Triggered off slider or back door open or by a switch. If you want to be able to control the lights and function as the factory cargo lights do, then buy a SPDT Center OFF switch. Locate it where convenient. Wire as per this diagram:
You can wire to the connector over the slider or to the connector over the rear doors. This switch and light setup will REPLACE the lights that came with the van.
To add light to the existing cargo lights and have 'em act like the existing ones, you would have to solder wires to the tabs where one of the light bulbs is clipped in, and be sure to observe + and - for LED lights.