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I have a OEM sub for sale if you want it. I know you can get it to play by using a third party amp like Mosconi, price around 220$ for the amp which gives the sub 200W, some more power than from OEM.
Unfortunately, you'd need to replace the MCU and add a stock amp. The older S's were wired for it - just slide the amp in and add the rear-deck speakers, and replace the front door speakers.
From personal experience, I can tell you that just upgrading the front door speakers alone with the ones from a Premium sound package make a huge difference. It's easy and inexpensive to do and keeps things "stock". (If you do this, you need to find the speakers and the speaker bracket as the Premium speakers are larger and won't just drop into the existing bracket.)
Sliding JUST the amp in without the MCU will not work as the audio signals are not present.