Hello Dan1935. Welcome to the Apple Discussions!
First, verify that your G5 "recognizes" the AirPort Extreme Card, by using System Profiler and note that the card shows up as the Wireless Card Type under AirPort Card. If it does not, then something may be wrong with either the card or the socket on the system board.
If it does, try resetting the
PRAM.
If that doesn't work, try resetting the open firmware.
o Start up into Open Firmware by pressing and holding the Command-Option-O-F key combination during startup.
o At the Open Firmware prompt, type: reset-nvram, and then, press Return.
o At the Open Firmware prompt, type: reset-all, and then, press Return.