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I would like to purchase the back cover to replace my original. The wifi antenna in it has been broke. DO you know where I can purchase one of these?

So if I understand you correctly, your WiFi board is broken. In case of the iPhone, you would be looking for the communication board. I believe it handles GPS , Bluetooth, and WiFi. You can get it from here and I am sure other places as well. The instructions are here It is not cheap, nor easy, so good luck to you.