Good idea using the adapter/outlet in the trunk. I put the pig tail connector that came with my tender directly on the battery and leave it hanging out on the carpet. Plug the tender into that, flip the latch like you showed and I'm all set.There are better solutions. I use a Sears maintainer with an AC adaptor from AutoZone. I soldered flat spade connectors to the AC adaptor as shown.
Just plug the adaptor into the trunk accessory plug. Note the trunk is full of golf clubs, etc.
Make sure you "close" the trunk latch to turn off the trunk light. I simply use the car key as shown.