I've done this before on many cars. What amp exactly depends on your auditory wants. Personally i'd go over to onlinecarstereo.com and pick something up from the sale section. If all you want is some tunes any old amp will do.
I'd imagine something like this would fit:
Alpine MRV-F300 300W 4-Channel V-Power Series Car Audio Amplifier at Onlinecarstereo.com
I'm hoping to do something similar when I start working on my interior.