I had my stock radio and speakers replace just a few weeks ago. I went to one guy and he told me all i needed was a powered sub in the rear. I ended up going to Tweeters and had the whole system replaced. Ended up with
Pioneer 5600MP head unit
Eclipse 5CH amp mounted on the rear of rear passenger seat
Alpine 6.5 component speakers for the front
Alpine 6X9 Rear
JL W0124 sub

Sounds great, Im glad I made the purchase