The "converter" mentioned by the dealer is actually a satellite radio which your truck is prewired for. The connector is dangling just above the brake pedal. If your purchased the "converter" plug in into the prewired cable and run the antenna lead you would be good to go. The 'SAT' button works with this radio. Dealer cost is in the $400 range plus installation, so $600 quoted by the dealer would be in the correct ball park.
However, you have an aftermarket radio with account already. I would use that instead of the Nissan satellite radio. You will not use the SAT button, instead you use the AUX position (just as you do with your ipod - the difference is you plug in the satellite radio instead of the ipod). You can obtain switched and constant DC power from the cable referenced above, connect ground to a convenient screw or bolt that goes into the chassis. I put the antenna for my aftermarket radio on top of the rearview mirror using a short piece of double sided tape. Route the antenna lead under the headliner, down the side window post, then to the receiver location.
