Well, both options are good. If you are keeping the stock speakers in there, just buy a cheap CD player head unit and pop out the old one. Almost all head units out now will have more than enough power to run your speakers. Ford head units are pre-amped, so you would have to get the whole unit replaced if it turns out to be anything but a fuse.
But, if you wanted to get a cheap amp, like a BOSS or Legacy, you would only need a 150watt amp. Then you just run a couple wires from the head unit you have now to the amp, and wire the speakers into the amp. You will get better sound quality and deeper bass, even from stock speakers. If you go this route, and want a wiring diagram/instructions, let me know with a PM.