when cargo doors are opened what activates the cargo lights to come on

The switch is probably contained in the door latch inside the door.

no wires in door latch. I am wondering if it is done by magnets

Look to see if there are any contacts maybe at the hinge side of the door. Are you sure there are no wires to the door latch inside the door?

there are no wires to the side barn doors. the back door wiring is for the licence plate light and the back up camera. I should point out that I am a licenced electrician.

it is in the latch. To test this: Turn on the ignition switch. Open the cargo door, use a screwdriver or other tool to close the latch fully with the door still open. If the "cargo door open" indicator goes out then your problem is in the latch and the sensor must be in the latch also. Can be remedied with penetrating oil sprayed into the latch. Owner of 2008 Express Van