Most vehicles these days don't have continuous power to the lighter/auxiliary socket - it's on while the car is on, and off when it isn't. What you could try is using a hardwired SAE cable used by battery maintainers, with a maintainer lead spliced onto the solar panel so it can be plugged in as the maintainer. If you run the car side of that lead through a small hole in the charge port compartment, you can plug it in without having to open the hood. Just make sure the panel has a blocking diode to prevent it from draining the battery at night, and consider getting a battery maintainer that uses the same SAE cable, as a backup. Finally, make sure that the solar maintainer is actually a good one.
Scarlet Ember 2018 Leaf SL W/ Pro Pilot
2009 Vectrix VX-1 W/18 Leaf modules, & 3 EZIP E-bicycles.
BAFX OBDII Dongle
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.