Check your ground, or move your ground wire to another part of the chassis.
I had this same problem, and I relocated my ground wire to another spot, it worked!
That 21'' Pyle is mostly for home theaters, I would buy a 15'' woofer, maybe a Sundown Audio SA-15, and is a little cheaper; and has more bang for your buck. But, you would have to get a amp, you can't run off your head unit...