Open the menu for the unit, press the menu button till you get to FMTX. From there you have a choice of between 88.7 to 90.1 that you can select. Pick one where there is no known FM station broadcasting and set it. You should now be able to select that freq on your van tuner.
You need to buy a FM Modulator so you can transfer sound to your FM Radio system.
See in the manual if theres a way to retune the wireless system most all units do have a little control someware where you can put your cars stereo on to a station with nothing on it and then you simply tune and tweek the adjustment so it sounds good. if you dont see it go to where you bought it or contact the service dept in the owners manual. Good luck
Usually answered in minutes!