Thanks for the tag
@jjrandorin
The SR+ and RWD lack all supporting hardware for the premium system with the exception of door tweeters, pillar speakers and in older cars the rear deck mids. The aftermarket harnesses from Hansshow and others often referred to as speaker activation harnesses power the door tweeters and pillar speakers by paralleling them with the left/right dash midrange, and likely the rear deck speakers with the rear door mids on older cars. (As a '23 RWD you won't have the rear deck speakers anyway)
I also have a '23 model 3 RWD and have done many audio upgrades including adding the factory subwoofer enclosure with an aftermarket subwoofer and amp. The factory wiring for the rear amp simply doesn't exist as far as I know, and even if it did it likely wouldn't simply work as the audio software running on a RWD is slightly different than the Premium system.
The factory premium subwoofer enclosure will bolt into a RWD (You can buy this from Tesla, or from Ebay/Salvage yard etc. just watch out for broken mounting tabs). You could then use the factory woofer, or use an adapter to bolt in an aftermarket subwoofer. You will need an aftermarket amp, I've designed a bracket that bolts onto the factory enclosure to mount the Kicker key 500.1 I also have subwoofer adapters for the Kicker L7T8 and Alpine 8". You can download the files for free from my website and 3D print them.
www.rwaudio.com
I used the hansshow speaker activation harness mentioned above to get the signal for the door mids, which is what I used to feed the signal into the Kicker amp that powers the sub. Power and ground for the amp are available under the rear seat on the passenger side. There is no easily available remote turn on (switched ignition) wire in the Tesla so using an amp like the Kicker with signal sensing to turn on saves you some headaches, also be aware the car has a 16v electrical system, not 12v as is typical so be careful what amp you use. The kicker is rated up to 16v making that part easy too.
Adding the subwoofer is a worthy upgrade, what kind of speakers did you use in the cabin?