Hi
I got a same story like you.I was opened the sub and look on the internet to he details and on the and I can now connect to any universal system.
actually it is verry simple circuit what is inside first wat its need to look the IC -its the tda 7396 (all of the data can found on the internet). so first its need to find the audio input pins on the connector of the sub so fallow the 1. and the 2. leg from the ic
then the second thing what its needed to look is the standby leg this is the 8. leg. It's need to connect to the +Vs point this are for examle the 3. and the 9. leg. it's can to bridge it verry esaly but also it's possible to put one switch.
My personal test was so:
It's works perfectly but like on the picture it can see that I need to repair some solderings..and that 2 little edittings under the connector is also my repair because my panel is demaged....
so now I will make it normally and then it will look normally..
I hope that I was help how to " hack " the bose 321 subwoofer...