For the Sirius text to come thru the nav, they will have to take apart the dash and wire it in. Don't know if you want to tackle that.
I just had the dealer do it as I am not about to take the dash apart on my brand new car. This way it is also their responsibility. The way I look at, the layout is close to $400 for the hardware. For another $400 on a car I'm already into for the mid-30s, I'd rather just have them do it. This way if any problems arise or they mar the dash or anything else inside, they can take care of it.