The stock amp is underneath the rear parcell shelf in the boot. Open the boot up and stick ya head in there and thats where it would be if ya got one.
As for speakers it depends again on the vehicle in question really like the amp. It maybe driving just front or rear or both.
Installing a new amp/sub is pretty easy, just time takes time, common sence and some patience really. Give us the heads up on what you want to do and im sure that you will get plent of help for what you want to do