Retained Accessory Power

Can any one tell me where I can find RAP in my 2003 GMC truck, it has a in dash RDS 6 disc changer.

I need the RAP to power my XM sat receiver, I have been to all the usual places and no luck, I have been told that it is not possible to get RAP as all accesorys are powered of the bat and then are turned on or off by the BCM or door modules.

I have also been told that the guys at PAC make a interface cable for an aux audio in that also provides RAP.

Does any one have what kind of a signal there is on the data buss line.

Thanks

Chuck