You have one of two things goin on......
I have not seen it on the superduties, but the radio problem you are having was common on the taurus...it was caused by water intruesion on the transmission range sensor that is mounted on the drivers side of the tranny,...its the odd shaped black thing that the shift cable is hooked up by.... this caused a short on a wire and made the Gem module do wierd things with the radio....
BUT...the TR sensor might be alright and you could also have water intrusion into the gem module, which is mounted to the back of the interior fuse panel........i have seen leaking winshields cause similar strange things. I would pull the that all apart and check for any signs of water intrusion into the fuse panel or gem module. if there is intrusion, try leaving them out to dry and then re installing them.....they might be ok....but if not, you will at least probably need a gem module and there is also the possibility you could need a fuse panel......