I was looking to add a battery tender as well and ended up just removing the interior on that side of the trunk. The red plastic cover has a couple of small plastic teeth/latches that snap in place around the plastic body. I had to use a mini flathead to undo the teeth in one place and the rest of it came loose. Even knowing how to remove the plastic, there's really not enough room/slack to do much. If I had to do it again, I'd probably just remove the interior cover again.