Dang!
OK, well it's a sunvisor light from a scrapyard VW that was mounted in the headliner above/behind the sunvisors (I think this was from a Passat, but many VW's have them). Cost me $5 from the local You-pull-it, but you can get them - albeit, more expensive - off eBay. here's an example:
https://www.ebay.com (commissions earned)
It's placed over at the side, because that's where there's space above the Mazda's headliner with a cutout where the wires are routed into the rear door. And yes, I simply extended the wires and ran them across and down & attached them to the wires supplying the side trunk light.
I also replaced the old festoon bulb in the VW light (which is quite dim) for a couple of strips of 5050 LEDs.
More photos:
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