Is an amplifier necessary for a car subwoofer?

You can buy one of those powered subwoofers where you run the power wires and speaker cables straight to the subwoofer, but they do not put out as much and not as accurate. All other subwoofers do require some sort of amp. If you try and power the subwoofer by just splicing off the rear speakers like the guy below me has suggested you will eventually mess something up. Subwoofers are designed to receive a certain amount of power, and you can blow them by either over powering them like too big of an amp, or under powering them like no amp at all.

Depending on what head-unit you have you mite be able to connect it directly to the aux in the back, but you would lose a lot of quality. You could also buy an active subwoofer which is self powered and therefore wouldn't need an amp. Active subs sound better too.

You don't really need one because if you splice the sub into the rear speakers it will work but it will basically be a 12" speaker in your car. They do sound better with an amp, you could pick up a smaller amp that will give you a comfortable bass sound and not the block shaking bass