You have all of the correct parts (your former -399 radio is correct for Sirius as well). Try these two things:

First, go into your radio's service menu and make sure the radio is in USA mode. This is how you do that:

* Turn on your car
* If the radio is on, turn it off
* Turn the radio on
* Immediatly push and hold the SELECT button until the radio serial number shows (5 to 10 seconds)
* Use the < > keys to cycle through the menu until you see the AREA mode. The radio will likely be set to NOAREA.
* Use the 1 or 2 buttons to switch to different areas-- set it to US.
* Turn the radio off to save the settings
* Turn the radio back on and use it as normal.

Second, make sure that your nav computer is updated with the latest OS. Sirius will not work if you have an early OS release on your MKIV.