Use a cheap OEM used subwoofer which comes out of the last 10 years generations of Suabrus with a aftermarket head unit. These subs perform well and are very easy to come by.
How it works
The OEM sub gets signal from the rear speakers, and power from the power off the back of the radio. You don't have to fool with switching wires or triggers to turn it on, its all automatic. It has its own internal amp as well.
The OEM harness has several colored wires, below is their colors matched up with a Clarion radio. Simply use a knife to strip a tiny part of the harness in the car, and splice the sub harness into it. It worked perfectly on my first try. I had to test the wires on the sub harness for which gray was which, and unfortunately when I figured it out I didn't write it down.