First of all, I'd like to say I've had Sirius for a year now....again, best thing I ever invested in. I don't even listen to FM anymore...
Now...for all your Sirius needs get them from:
TSS-RADIO
They are the best.
2nd...IMO, and from what I have read over and over again, the factory units are just OK...the sound quality is not as good as the plug and play units. The 2nd advantage of a plug and play is that you can take them inside, hook them to your stereo, or even put in a boombox.
Do some research 1st. You can get a plug and play unit, for the same price as the factory unit that gets put under the seat. They all come with the suction cup mounts, but they make custom brackets, that hook to the right side of your dash, hides all the wires, and keeps it nice and neat..
Also....does your stereo have an AUX IN?? For a plug and play, this is the best way to hook them up.
If not, they make an adapter for your FM antenna (which you will never see, gets put behind your head unit) without this, Sirius isn't worth getting because the FM Transmitters in the newer radios suck.
I have a Sportster Replay in both my vehicles.....I love it.
Any More questions, ask away!!!