Subwoofer/Amp from factory sub wires

PF5150 Newbie
Nov 8, 2020
Needing some help with the install of a sub/box. Have an older four channel Alpine amp powering a single sub. 2011 Jeep Grand Cherokee/stock head unit.

So, the Jeep has a factory sub in the back and has two sets of speaker wires going to it, a single driver. I have assumed that they are simply a L/R speaker-level output, but I'm not clear on how two channels (4 wires) can be wired to a single driver. That's problem #1, I'm not sure what kind of setup this is.

Assuming anyway that these are regular L/R pairs, I connected them to the Amps's channel 1&2 speaker-level inputs. The sub output was low at high volume, but at lower volume the sub output is too high. I reasoned that it might be out of phase so I inverted the speaker wires (no phase option on this amp). This helped bass but sub output continued to be too high at low volumes. If I turned down the gain, output at higher volume would still suffer. Bass at this stage is ok, not great overall.

While playing with the speaker to Amp connections, I saw that if channel 1 was connected and I connected just one wire of channel 2 to the amp, the sub would come to life and I had to turn down the gain because it was rattling the panels in the car. How can just one wire from a pair do anything? I can't see how this could be good for long so that's problem #2.

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