Amp Wiring - Do I Have This Right?

There are probably different variables. In my case, I have the amplifier in line with the speaker harness, ie, it goes head unit -> amp> - out to speakers. I then have the subwoofer wired in to the rear speakers. If I turn the fader on the amp "ON," ie, so it's processing both front and rear inputs, it completely cuts bass to the subwoofer. If I turn it OFF so it's only processing the front inputs, the subwoofer works fine. My thinking is that even having turned off all DSP/signal processing/etc via FORScan, there's some bass roll-off happening in the rear speakers.

So, I'm going to try tapping into the front speaker kick panels first, since that's only an extra 2-3 feet of wire, and if that doesn't work I'll just tap the subwoofer into the speaker wires going to the input of the amp, bypassing the amp's built-in DSP.