The same system is in base models, as well as the upgraded, but all speakers do not function on base models. Including sub. The super easy way to fix this is....look up the amp for upgraded system (ltz) models and swap amps. It's plug and play exactly. Opens all speaker channels. It's not a loud improvement but sound quality is better. Find part number and look on ebay. That's all I did in my 15 terrain sle1
I can vouch for this. Below is the description of what I got off eBay for a little over $100. It took about 45 minutes to install and the factory sub was fully functional.