Weird stereo issues

[BozB98]

Hello!I apologize, this might be a bit long. First of all the car is a '98 avant with the Bose system, but i bought it with an aftermarket stereo already installed. I've never had any sound from the tweeters or the subwoofer, only the 4 big door speakers ever made sound. I understand, that the bose system uses a single central amp to drive all speakers, so I always assumed that they just hotwired the 4 door speakers and left out the rest. Now that the fronts are on their way out, I decided to swap them and started checking the wiring, and this is where the funny things are... They've used a Bose to ISO adapter, with the little black box that holds what looks like resistors and capacitors, and a ground wire that only comes out on one end and wasn't attached to anything (wire on the car side connector comes back to "low frequency ground"). The main speaker wires of the HU are attached to two different connectors as per the adapter (?) and the 4 positives for the LF,LR,RF,RR are wired to the yellow 6 pin. Pictures below:

[BozB98]

Sorry couldn't attach pictures, but it's pretty plug'n'play bose/ISO adapter.

Now I figured that that ground was there for a reason, so I grounded it on the chassis of the HU and to my surprise the subwoofer roared to life, but the rear door speakers went completely silent other than some noise. As soon as I remove the ground wire, the subwoofer stops and the rears start sounding again.

If I turn the fader to the rear, all sound stops whatsoever, in both cases. Tweeters never make a sound either way. No RCA is used whatsoever, only the HU speaker outs.

What could be the situation here? Some speakers are wired directly to HU? I can't make any sense of this.

Thanks!