If you don't have an SMB interior and your van is pre-can bus (What year is your van?) then I can just about guarantee they tapped off the original dome light wiring.
Pop the buttons off the trim around the pop top and start removing screws to remove the trim pieces on the sides and front and rear. First. you'll have to remove the front and rear latch plates in the center of the fore and aft panels that run crossways, one above B pillar, one above rear door. Then it's just 4 simple upholstered trim panels finishing out the pop top hole. You may have to peel some carpet/upholstery back and re-glue it after. Can't remember. This will expose a lot of stuff to you, the springs and mechanism which are something you might want to see but also the wiring on the left side.
From your lights in the pop-top a pigtail comes down in one corner of the top, can't remember where SMB drops it, probably in the rear left. It either goes across to your original dome light or joins the main body wiring loom about midships where the center dome light used to come to the middle of the factory headliner. If neither of these places it will be in the B pillar behind the trim somewhere between the pillar and the front seat center dome light
It's probably a simple splice in there with cheap speaker wire and an inline fuse. There's nothing impressive about the way old SMB's were put together.
Once again, year model could help us tell you more.
AWOL
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