Crew vans come with interior (courtesy) lights in a line down the center of the ceiling. We removed the headliner, so they were dangling loose for a while. We didn’t want to cut holes in our new ceiling for them in the same place, so we moved them.
The biggest issue we thought we’d face is that the light closest to the front of the vehicle has a three-way switch on it: On-when-doors-open, off, on-all-the-time. Without that switch, the lights just wouldn’t function.
Poking around in the van when we were running cables, we found that there’s an identical plug sitting loose behind a light-shaped cut-out in the metal above the rear doors.
It’s really unlike Mercedes to provide any additional plugs on the wiring harness, so we didn’t know what this one might be for. However, it had the same color wires and, miracle of miracles, when I plugged the switched light in to it all the same functions worked.
Above the sliding door there’s a bolted-on metal frame that also has a cut-out for a light fixture and a regular two-wire supply plug sitting loose right behind it. I tested it with a voltmeter and it, too, is a spare light location with power already wired in.
We’ll probably use just those two locations for courtesy lights in the van. We only need enough light when the doors open to be able to find the switches for the lights we’re installing. In the future, I might even re-purpose the light harness above the sliding door to power an LED strip along the step instead.
I’ll buy a couple of LED bulbs to replace the incandescents that are in there at the moment, but otherwise we’ll leave those two locations stock.
Interestingly, there are holes pressed in to the steel all the way along the top of both walls which are also just the right shape for the lights to fit in to vertically. If we weren’t going to be covering over this area with our wall panels, it’s another potential location for more lighting.
The wire harness for all the lights runs along the top of the driver’s side wall, with tendrils off along each rib to reach the factory light locations in the headliner. If you unclipped each of these offshoot wires from their mounting points, you could run them to the wall locations instead of the ceiling location really easily.
Update: we replaced the incandescent bulbs in the dome lights with warm white LEDs from Amazon. So far they seem to be working well.