I had the same idea to put a sub & amp in my base model. And yes, went to an aftermarket shop & was quoted $$$$$$$ for a fiberglass enclosure job. So I ordered the subwoofer box from GMpartsdirect and got the same "backorder" reply. After about 6 business days I sent them an email asking on the status. Now i don't know if it was coincidence or if they actually read my email & took action, but i got the enclosure 2 days later!

After all that, I haven't had the time to get it installed yet, tho!