There was no factory ipod option for the 2006 XLR. It would seem someone installed an aftermarket mod (there are several available) to allow an ipod to interface with the stock infotainment system.
Recently, I wanted to interface an ipod with my XLR, and did a lot of research on the products available.
In one case, I contacted the vendor and tried to decipher what their ad really meant. They admitted the XLR and their product weren't compatible, even though their website said differently.
Another product has a bad habit of draining the battery when the car is shut down, so that was out. (Despite the built-in battery saver function mentioned in the owner's manual, your battery will eventually drain to the point of being unable to start the car if you let it sit long enough.)
At this time, I have yet to hear of a 100% ipod interface solution. 100% means full functionality with the car's displays and controls, just like the stock infotainment system. You can get about 75% functionality, but it isn't always cheap, or easy for everybody to implement.
My wife's 2011 Chevy Traverse has multiple USB ports and is truly plug-and- play. It's just treated as an aux input. I wish the XLR was as easy to interface with, but it is what it is.
I settled for the cheaper, low-tech approach, and bought an FM transmitter that plugs into the center console, under the arm rest. The quality is okay, but not as good as say, a CD. It's easy to remove, and I'm tempted to cancel my XM subscription, since they're about the same sound quality, and at least I have a choice about what I get to listen to.
One of the aftermarket ipod interfaces for the XLR requires the operator to press a series of buttons while in XM2 to get it to operate; maybe that's what was installed in your car.