Hi,
Its unbelievably easy to replace the Airport card in an ibook.
At the top of the keyboard there are to clips, slide them both back, lift the keyboard and fold it back onto the palm rest, take care not to damage the ribbon.
The airport an memory bay are both under the keyboard. If my memory is correct I think you have to unscrew the metal plate to get to the airport.
Try removing the airport and cleaning the connectors on the card and the socket on the logic board, the compressed air in a can should do the trick. if this doesnt work ebay is the best place for a new AP card!