XM Radio with 2004 Head Unit

Does anyone know if the GM factory head unit that is capable of receiving an XM radio signal is the same as the head unit you get with a vehicle that is ordered without the XM radio option? Basically I ordered a truck without the XM radio option and now I would like to get the service. The head unit looks identical to my buddies and he had XM installed. I'm wondering if it is as simple as getting an antenna, hooking it into the back of the radio and ordering up the service. If anyone has done such a thing could you please give me a little insight into the process?

Thanks

it's not that simple when u are using xm there is a actual reciver that u have to mount some where on the roof or near a windo i wouldn't recomend tackeling it ur self it's not that easy