Assuming you have a radio with the 6 CD changer in your Titan, then, yes, it is prewired. The Nissan XM (or Sirius) satelllite radio is a box which mounts under the dash on the drivers side. If you look under the dash, just above the brake pedal, there is a connector dangling there. That is the connector for the satellite radio. Other than physically mounting the box, plug in the prewired cable and the antenna lead and you are ready to go. You will need to position the antenna, not sure where it is being mounted these days - at one time it was mounted on the rear window, other mounting positions include just above the rearview mirror.
The factory radio is the 'slickest' install as it connects directly to the HU and uses the HU display, plus the steering wheel controls work automatically. The downside is cost - it is the most expensive satellite radio you can buy for an auto.
Since you have a radio already, I would install that instead of going the Nissan route. The connector I mentioned above contains both switched and constant DC power which can be tapped for the aftermarket radio (I used short pieces of solid wire soldered to the ends of the DC wiring to the radio, inserted the solid wires into the appropriate socket on the connector and taped in place). Audio from the satellite radio can be plugged in to the AUX audio port on the dash (one can obtain a right-angle adapter/cable at RadioShack) and the antenna can be mounted a number of different places. In one configuration, I mounted the antenna on the rear cabin roof and routed the lead-in through the brake light, presently, I have the antenna on top of the rearview mirror with the antenna wire routed under the head liner, down the drivers side windshield post to the floor and under the floor mat to a position under the driver's seat (where the satellite radio is located). The control head for the satellite radio is attached to the dash just above the A/C vent using double sided tape.
