Hi:
Without knowing the true history of this car, you are at a bit of a loss. My first thoughts are that perhaps a fuseable link is blown, that connects the alternator to the system.
To troubleshoot this car, you need real wiring schematics, and a good, cheap source for those are at:
www.alldatadiy.com
For about $20, you can get a subscription to your focus, and it will help you get it working the way it should be.
These subscriptions have allways worked for me and are very helpful on difficult projects.
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