We do not have the Technology Package on our TB, so it's the standard entertainment sound system. However, we do have a second sound system which is the Active Noise Cancellation (ANC comes standard with AWD). The ANC system should be playing back an inversion of the wave patters from road noise and engine noise, to reduce the interior perception of these sounds.
The standard entertainment sound system is specified to have 6 speakers. There are two speakers for each front door, and one speaker for each rear door, making the six. However, there are additional tiny speakers located at the interior handles above the front doors. I think that these speakers are part of the ANC sound system, and I suspect that the speaker inside the spare tire is also part of the ANC sound system. I doubt that Chevrolet would miss out on an opportunity to market the inclusion of a "subwoofer" with the entertainment sound system.
I spoke with the dealership manager where we purchased our Trailblazer and asked about the speaker under the spare. He said it's a subwoofer which is part of the Bose 7 speaker system. Basically it's the 7th speaker in the system instead of the base 4 or 6 speaker systems. If you remove the spare and play some music you can hear speaker playing the lower frequencies.
How does that explain the presence of this speaker in models without the Bose 7 speaker system? At this point, I have literally zero faith in the credibility of dealership knowledge for the '21 Trailblazer.
It is possible that both explanations are true, and the speaker is used for the ANC sound system as well as the entertainment sound system, concurrently.
The fact that the speaker plays music says it's at least part of the sound system and has to be a subwoofer since the frequencies it plays are low. It is also possible that it's part of the sound cancellation system, but I can't find any information to support it. I agree it's a good idea not to trust everything the dealership tells you since they all act like they know everything anyway, but it's clear they don't know much about the new Trailblazer.
It would be good to learn which vehicle configurations do not include the speaker under the spare tire.
Our Activ AWD has one despite not having the Bose sound system, but the speaker housing also has a suspicious sticker labelling it as a pre-production sample. Could our early Activ have been accidentally misconfigured at the factory, perhaps?