For my XM, I got a custom mounting bracket from Pro-Fit for about $40. And I got an adapter that makes the stock head unit think that the XM is a secondary CD changer. It was expensive ($265), but worth it. Here are the links for both.

http://www.pro-fit-intl.com/vsm.html

http://www.vaistech.com/slu.html

There was just enough space inside the console to put the unit inside right next to the radio. Using the FM modulator is the easiest way to do it, but also the worst sound quality...not to mention the fact you always have to change the frequency while traveling. Using the tape adapter is crap too, because for one, you have the stupid wire hanging there and there's the constant hiss from the tape player. Since the IS isnt XM ready, your only wired options are to cut into the factory harness or buy the adapter.

Here are a couple pics. Quality isnt great bc I took them with my phone, but you can get the idea.





Hope this helps