Here is the simplified install instructions referenced by wdavidsharp:
Photos of simple XM install
I initially installed my XM receiver piggy-back on the RF amp per Nissan's instructions and then later re-installed per schneid's instructions when I re-configured my stereo with aftermarket amps. I highly recommend doing it schneid's way. If the Sirius receiver connects to the N-Bus cable, you should be able to install it pretty easily using schneid's instructions.