How do i upgrade sound system??????????

Yeah, 2x10's in a box with an Amp will provide a boomy backseat. I would suggest replacing the stock speakers with high end aftermarket replacements or components and power them with a good 4 channel amp. This will improve the soundstage and balance out the boom coming from the back. Good Luck!

Get one L5 in the back of your stang, takes up less room and has alot more punch at higher volume and sounds better. Go to Circuit City and have them show you one, take your own CD, tell them to turn it up as loud as it will go.

Get a Panisonic CD dash, so far they are the best that I have used.

Kicker 400.1 Amp will provide enough juice to the L5 without blowing it and this combo will deff turn heads.

Either get infinity or Kicker speakers for your doors, ask the circuit city rep which he thinks would be best.

All the guys up at my circuit city are very knowledgeable, they might not be at your store though so watch what he says.

go for a nice pioneer head unit, best sound quality for the money IMO

also, stay away from best buy and circuit city, they are clueless and way over-priced.
www.millionbuy.com is a good place to get all your car audio equipment from. thats where myself and friends get our stuff. get a nice radio with 4 new 3-way speakers and buy the 460 tweeters and just wire them in. and then whatever sub and amp you want.