You can buy it from several outlets (search the Web for 'murano xm unit') for about $300 or so. Installing it doesn't look too difficult if you're moderately OK with tools.
A cheaper alternative is to buy an aftermarket XM unit, like a Roady. You can run it into your radio via an fm adapter or tape adapter, or buy a $80-90 adapter to run it directly into your stereo. Aftermarket units can also be removed from your car and brought into your house. I use a Roady.
I bought the roady xm kit purely because i am not planning to pay for 2 monthly xm subscription for 2 different units.
On weekdays, most of the time I am inside the office, so it makes sense to have the xm inside.
On weekends,i need the xm in the murano.
To have the best of both world, aftermarket is the way to go.
I have an 04, installed a Roady2, used the free register number here on this fourm, Installed the PAC system BUT at the rear without removing the radio, Everything is great I also hooked up my Ipod using the PAC unit and everything switches from the ABC presets on the radio, Look at the treads under Ipod and my and JAAKS posts on the wiring to make it work at the rear. GRIP
I mounted m antenna hidden under the access hatch on the rear hatch lid a search will show you pictures of my install and mattsmurano and others show the antenna installation mine is behind the shift selector near the boin box look under I pod mountins search GripperDON. Grip