Hi: We have a 2019 Smart Fortwo with the passion pkg, sport pkg and 535 Media System. I've added the std tweeter upgrade in the doors to the existing system and it works fine. I don't have the JBL system but would like to add rear speakers and a subwoofer. Does anyone know if this is possible with the existing system? The wiring for the tweeters was already in the doors but I don't believe that's true for either rear speakers or a rear subwoofer. I can't seem to find any helpful info on this possible upgrade. Thanks.