2006 A4 and satellite radio
 

So the question is this - They come "satellite ready." Does this mean
that I HAVE TO buy the Sirius unit from the dealer in order for it to
work thru the factory head unit? Or can I buy it from a Circuit
City/Best Buy and have them install it?

In other words, to run it through the factory head unit, am I stuck
paying way too much to the dealer, or do I have options?

Thanks in advance.