XM Radio with 2003 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

I also have an 03 and I'm thinking about adding XM. Turns out there are adapters sold by Soundgate and PIE that will allow a Pioneer XM radio unit to be controlled by the OEM head unit - including the steering wheel controls to if you have them.

There is good info on this subject at the following links.

http://dieselplace.com/forum/forum_posts.asp?TID=1416&KW=xm+radio&TPN=1

http://www.soundgate.com/products/cd/gmc.htm

http://www.xmfan.com/viewtopic.php?t=12127

:)

I have a new Silverado 2500 Crew cab. It did not have satellite radio. I just put in the XM Commander. Fits in nicely in the compartment below the radio. Uses an FM frequency. It is not as good of a quality as a direct satelite, but is definately worth the money for it. It sound great, and no commercials! Many choices of music, news, programs, ect..

Satelite radio is definetly worth the $9.99 I pay monthly. Whatever it takes to get XM is definetly worth it. You won't be dissapointed. I have the XM commander in my 2000 Z71.