2003 H2 Want To Add XM To Nav Radio

[Springfield H/2]

Need alittle guidance please. I have upgraded to a Factory Touchscreen Navigation Radio and have installed it and seems to be working just fine. It is XM ready and I understand the XM module would be above the glove box if the vehicle was so equipped originally. I am very positive the factory radio did not have XM. IF I purchase the XM module, is the wiring there to plug the module in too? And what are folks doing for the antenna? Is the antenna plug there for what is already on the roof? I have heard the XM shares the antenna with the On-Star system. Basicly what I am in need of is the wiring harness already there to add the XM module and plug for the antenna lead. I am open to any ideas or knowledge you each have experinced or know. Thanks for all the help on my previous post. We have now owned our H2 for a month and love it.