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I had a black 3GS I gave to my 13 y/o son when I got a 4S. The 3GS had cracks on the rear panel and the volume button came off. My son wanted it fixed or a new one. I had previously replaced the battery from iFixit 1 year ago and did not want to give up on it. If found a white back panel on iFixit. It was a little more expensive then other sites, but it was EXACT and required NO modifications.
I took his phone after he fell asleep on Christmas Eve with the intention of having it ready Christmas morning. Overall, it took 3 hours. 2 cups of cocoa, patience, and the illustrated Step by Step guide were a MUST. I finished and all went well except for a no SIM status after install. Did some research and got it done and it looks brand new. On Christmas morning my son couldn't even tell if it was his own phone. He loves it and is happy that the white 3GS is somewhat unique. The 3GS is perfect for a 13 y/o and will hopefully last a long time.
I have exchanged several batteries now. I recommend a piece of paper where the removed screws are set on and lightly held down by tape and labeled so none are lost or mixed up. Also, the mother board should be pushed slightly to the top of the device before tightening the motherboard screws to the case. This allows for the sim card to push against the SIM card detector. Lastly, the back case includes the silver trim and volume buttons. If the glass is in good shape, it will look brand new when done.