I have installed simple AAA battery operated LED puck lights in the lids of my saddle bags with hook and loop tape. They can be removed to be used for other purposes. Such lights are available at any big building store, Lowes, Home Depot, etc. A cheep and easy solution...
Harbor Freight has those blue LED lights that are usually free, which have a large LED area light and magnet on back plus a hook to use while on the road or the like. It also has a small flashlight on the edge. You could mount a piece of metal under the trunk lid (especially if you have a trunk rack) and use the magnet to hold it.
That being said, I do have a pair of small lights each side that are switch operated....but only when the bike is on I believe....wired into tail light circuit. I don't have the trunk mounted up right now, but it is on the shelf and I could take pics if ya want. But, beings it is not wired up to battery for constant power, I still use that little blue HF light when the trunk is on.
Something to consider is a fault with that light where it might stay on while bike is off and parked and draw battery down. Plus if you wire it up and want to consider Super Valk options, it would require another plug if wired to constant power.
A friend had a LED wired up in his trunk, with constant power in line with a power port for phone charging in the trunk. The light was operated off a switch you might find similar to a refrigerator or freezer door, and turned on when the lid was opened.