toyota jbl shared ground

I have a 2003 Toyota Avalon.  The factory headunit has 5 wires going to the factory JBL amplifier.  One wire for each channel and a "SIG GND" wire.  I think the "SIG GND" is a shared ground.  After the amplifier the speaker wires for each channel have both a negative and positive wires.  I have a JL audio Cleansweep I want to install.  Is there any way to install the cleansweep before the amplifier?  The cleansweep has a positive and negative lead for each channel, but there is only one wire per channel coming from the head unit.  I tried connecting all the cleansweeps negative input wires to the shared ground from the head unit but the cleansweep said it was not receiving any signal.  I tried measuring the output from the head unit with a multimeter but could not get any reading.  I would like to avoid installing the cleansweep after the amplifier because 1) I planned on using the space under the seat for the cleansweep and 2) I cut the amplifier wires too close to the wiring harness and it will be difficult to hook the amplifier back up.  I can e-mail the wiring diagram.

Kyle