Ummmmm, unless theres something I dont know, which is likely, these gen 2 sd's are getting up in age and mileage and the cabs are starting to leak. Same cabs as the 99-04 trucks. Which all usually have leak issues at the windshield, back window, and cab lights.
In our trucks it usually wipes out the gem and then the ac starts blowing out the defroster, you lose reverse lights, and sometimez the rear cab light and or the interior lights will stay on. Often, in the early stages anyway, it would stop when you started the truck and turned lights on or touched the brakes pedal. In extreme cases it'll short out enough circuits that your starter will begin to run and keep spinning until the batteries go dead.
Fun shti.