Black screen but still working?

A friend of mine give me is old iphone 3gs telling me that is dead. The glass is broken bad and nothing on the LCD screen. Back home i just plug it in the computer and surprise Itune recognise the divice and everything was there. If i plug the iphone on the dock station on my radio it says connect but impossible to start the music. With the usb cable on my car radio it says connect and if i make a search i see song name but when i press play nothing happen.

Unplugged the divice and start playing with it (trying to touch the icon by memories) but nothing happen, no sound, no back light excepte when i put the lock button on it vibe like normal (well it is normal ??? never got an iphone before).

Firs of all did i need all the screew in place to make the divice working normally ??? ( no screw holding the lcd screen to the digital glass and no screw on each side of the dock connector he lost those)

and second ..do you thing that if i change the lcd screen and the digital glass and the speeker / home frame it will work again normaly ??? or i only wast money and time. (because right now theres no speeker install cuz he take it out and lost it)

Thanks

Finaly got all the piece and replace them and all works fine ..........well almost.