I just realized that you are talking about the REAR switched dome light and not the front. If your 18' is like my 07', the switch has 4 wires as follows:
RED = +12VDC CONSTANT ON sourced from fuse #19 @ 7.5amps. This is "common" for the switch AND bulb. Meaning the bulb always has voltage and only turns ON when it is "sinked" to ground (-) by canbus or auxiliary "aftermarket" switch (added by you). The factory switch does not turn on the light directly, but tells canbus to close the ground circuit (Brown/Blue) which then completes the circuit and illuminates the bulb...
Brown/Blue = SWITCHED (-) GROUND, meaning this ground closure is controlled by canbus based on the input signal from switch, and vehicle status. This is the wire that actually controls the light.)
Violet/Green = input signal (+12) from switch TO canbus for standard ON command.
Violet/Blue = input signal (+12) from switch TO canbus for "DOOR ON" command.
A simple way to command cargo lights ON is to put add a switch to "short" the Brown/Blue wire to ground. This can be done anywhere in the van. The wire harness runs near the roof on the drivers side and all wires can be found there. You can "create" a ground by threading a screw into the vans body to attach a wire or tap/add to one of several factory grounding points. When this wire closes to the Brown/Blue wire via switch, lights will be forced on regardless of rear door switch position. This method keeps full functionality of the light control when the new auxiliary switch is off.
EDIT: When I tried to incorporate the canbus logic, I was not satisfied with the "timeout" feature and used the above work around.