Hi from Germany,
There is a video on youtube to hook up the Subwoofersignal. I also try it, but there you cant find a real Stereosignal, it is already crossed. If you hear Music with drums from left side to right side, you dont hear it because a subwoofer add all deep sounds and give it out in one Speaker. Subwoofer are only good for Cinemasound. Music is always in Stereo.
But you get a better sound when you hook up the rear door speaker signal and add an amp with a possibility to turn on 12 db more bass. So I did. I also changed the rear door Speaker one Size bigger to 16,5 cm. I needed a sharp knife to cut the plastic around. This mod was cheap - the bass speaker around 20 Dollars, a used crunch blackmaxx on ebay also 20 Dollars.
And If I find a way to throw out the Bose amp (a Joke, not an amp) , incl. the "Subwoofer" and get the onstar Signal to a separate speaker, I will search for small 4 chanel amp which I can mount on the place from the bose. The "Bose" mostly "Delco Bose" was the first thing I throw out in every car that I had - for example Corvette C4 , BMW 850. The Bose in cars you can not compare with the legendary Soundsystems from earlier times. (Sorry for my bad english.)
Greetings Michael