Before I go on, I take no responsibility for what you do to your car
I wanted to add direct input for the iPOD to the wife'* 2004 LeSabre, because the FM modulator type of input is crap.
(Why GM couldn't have added an aux mic type input in a 21st century car is beyond my comprehension, but I digress)
I bought an iPAC-OEM and AB-GM10 harness, designed to tap into the XM system:
Two problems: I found that this harness is not compatible with '04 LeSabre (and PAC does not make one that is), and even if it were compatible it is designed to effectively replace
the XM receiver box and would thus not allow the use of XM.
Anyway, here is my XM receiver box (trunk, driver'* side, behind the mat trunk liner):
For reference, it says "Delphi Delco Electronics part number 25762447" on it.
I used mini terminal blocks from Radio Shack to make sure my wiring would be secure. With help from hawkjet and an FSM, I figured out what wires to tap into:
-->XM harness in car
Red-->Brown/wht-->Right + audio
White-->Green/wht-->Left + audio
Orange-->Black/wht-->Common - audio
Black (thick/"dull" insulation)-->Bare-->Drain (prevent ground loops, I think)
Black (thin/"shiny" insulation)-->Purple-->Communications
Now then, to do this I had to cut off one of the connectors on the AB-GM10 (see red arrow in second pic above), AND I had to sever the harness connector going into the XM receiver (and strip back some insulation on all these wires). For compactness, I cut the 8-gang terminal block in half and wire-tied it together to make a 2 by 4 array (rather than a 1 by 8 array).
The XM harness wires are "reconnected" through the terminal block. The AB-GM10 wires then go into one side (doesn't matter which). This way, you still have full XM functionality, and all you have to do to remove the whole iPAC-OEM thing is pull the AB-GM10 wires out of the terminal block.
Note that this does NOT give me head unit control of the iPOD. The iPOD connects to the long wire that comes with the iPAC-OEM (see first pic above), that wire stretches into the cabin, and the iPOD is controlled by the iPOD'* own controls. It does charge, though.
To listen to iPOD, I turn on XM radio as source, and tune to the "Radio ID" channel ("blank" channel). The HU display shows some wacky crap like "XYYXZZ"
All the other XM radio channels seem to work as they should. The iPOD will "play over" them if you don't shut it off, though
Total cost, $100 to $120, depending where you buy.