There are plenty of small amps: Cambscar did the install into my miata (removing the old mp3mobile - after they'd had a play with it ;) ) and ended up using two of the smallest, cheapest Alpine amps - two because there wasn't anywhere neat to put a 4-channel amp, so two 2-channel ones were used (I didn't want to lose any trunk space).

The amps were MRP-T306's, I think they're rated at 30W RMS x 2. Plenty loud enough for a small car.

Hugo