I have a 2006 RTL with sunroof and NO NAVI. I have OEM XM radio in my head unit.I'm not sure what model your 'new' one came from, but I believe XM was only available on 2006 models with Sunroof & Navi. In those models, the XM receiver is located behind the back seat. They do indeed hook up to the roof mounted antenna. If that's the case, maybe you can get him to pull the seats/panel & retrieve the XM receiver for you to work with.
I don't know about the XM "Kits" at all (dealer installed), but you can get them at Handa.com, antenna & all.
If the 'new' one came from a 2006 RTL with factory XM install, then it will have a navigation display screen (only available on Sunroof/Navi models).
If it is NOT from a navi model, then it will not have had the XM from the factory (would be "XM-ready" instead), but would rather be subject to the dealer kit. How aftermarket kits compare to those dealer "accessory" kits, I haven't a clue (sorry).
Anyway, I'm fairly certain the above is correct, but I'm sure you (& I) will hear if it is not. I don't know how things were offered after 2006 (I know they all had sunroofs!), but maybe you want to find out what year & model your new radio came out of.