I'm not sure I'd tap into something nearby like the tail lamps. Better I think to have a dedicated line/fuse for it but I'm not knowledgeable enough to know if it matters either way. I would suggest some research on YouTubers installing those things to glean some info on best practices, or maybe a quick call to an installing place? Play dumb, say you are considering a device but aren't sure if an EV is able to hook something up in the rear? They'll either say, sure we do it all the time, or no, a separate connection is needed for whatever reason.
