First I would replace the DC-In board. Here's how: [guide|255]
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Here's a source for the part: http://www.ebay.com/itm/New-Apple-PowerBook-G4-Aluminum-17-DC-in-USB-Power-Board-820-1388-A-922-5769-/151765309096?hash=item2355ead6a8:g:JgAAAOSwiCRUlzwj
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Next go into the system profiler and see if the airport card is being seen. Hook up to a hard wired ethernet cable and see if it will log on to the internet. Let us know your results.