amp with factory infinity system

I have a 2002 Chrysler Town & Country with factory Infinity stereo (I think it's factory upgraded w/seperate tweets in back and front). I want to add a Kenwood amp running 2ea 10" Kenwood subs (dual voice coils), and tap off of the factory head unit for the amp A & B channel incoming signals. The amp is a 4 channel, bridgeable to 2 channel-4 ohms, which I have wired as such now. What factory speaker wires can I tap into for good signal from the head unit to the amp? I ran 4pr/18 gauge speaker wires from what I thought was a good signal from the long rectangular connector on back of the head unit but when I ran the system up the factory speakers clipped a couple of times then cut out altogether, leaving the subwoofers to play alone. I pulled the power to the amp and the factory speakers came back on. There is another, square, plug on back of the head unit, does that have speaker wires that I can tap into for signal to the amp with no issues?

Thanks, I looked under the seat, nothing but power seat stuff and a heater duct for the back seat.  I think the factory amp is built into the head unit-it has a huge heatsink attached to it.  Anybody else with any ideas/suggestions?

Some of the Infinity systems have the amplifiers built onto the speakers.