If it's the 2015, that isn't the Beats system. That was first available in the 2016. The 9 speaker system has a sub in the back. There is a crossover built in that connects to this. If you are planning to put in a new system, you'll want to include a new crossover, potentially a new sub, and of course 9 new speakers. When you bought the vehicle did you ask for the 9 speaker system? It wasn't standard, and I'm not sure it came with the 5.0 head in the Limited.
In order for us to help, we need to know you have a speaker in the trunk. Best way to confirm, remove the grill in the trunk and see of you are staring at a speaker.