The higher option Explorers have a battery saver, which shuts down the lights after about 20 minutes or so. My 93 Limited had it, that was slick back then.
The solar panels available these days got me looking for a charging method with them. I bought a solar panel controller, from Battery Tender, for about $22. I don't have a panel yet, I just measured that the ideal size would be close to 12x28", to fit the sunroof. Assuming a panel is available to fit well, it should trickle charge a battery well, like the home units.
Hunt for that panel, see if there's an off the shelf version. It would need to be thin, the flexible type are typically very thin with the thickest point being the electrical connectors attached.