I want to thank the different people who answered.

As to why not an amp and all:
1) I'm buying 40W headrest 2" by 3" speakers, the best ones you can get for that
size (so no, don't think I'm using the crappy factory speakers)
I'm not quite sure what a 200W amp would add to speakers that can't deliver
that much in the first place
2) I have a small car, with a not very powerful battery. Not only do I have
little space for the amp, but it wouldn't run very long with the engine not
running.
3) I have another, bigger car, and that one has an amp.

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