This is normally done as a "Built to Order" option at Apple. But there may be a case that this applies to you.
Do make sure you follow anti-static guidelines in your G5 manual before reaching inside any computer.
Did you get the original "inside install" Bluetooth module for your G5? If so, you need to do the following.
1) Unplug the machine. Lay it on its side with the door up (a soft cloth under it is nice)
2) Open the door. Remove the plastic cover. Locate the speaker/fan panel in the upper front quadrant.
3) Pull on it gently to dislodge it. There are wires connected to it that plug into a small connector on the mother board. Unplug this and remove the fan/speaker.
4) Look at your B/T module. There is a small male connector on the board. There is a matching female connector on the mother board above the RAMS. There are two small black wires, actually miniature coax cables, near here. One is plugged into your airport card, if you have one. The smaller of the two has a teenie, weenie connector on it that must be plugged into the blutooth card and firmly seated.
5) plug your bloothoth module into the connector on the board aligned with the holes matching the threaded slots below. Secure it in place with 2 #4-40 x 1/4" screws. (These might be #2-56 x 1/4, I don't remember now)
6) Put it all back together, remembering to plug in the connector on the fan/speaker board.
7) Close it up, set it up, and try your bluetooth out.
Understand why Apple makes this Factory Only.
If you module has a USB connector on it instead, just plug it into any available USB port or hub. (example D-Link)
-Ron Burns1 (as known on Apple Support Forums)