hey guys, just wanted to get some views on an issue we have been having with our truck. we have an 02 2500hd with the regular single zone hvac controls. about a month ago it stopped working on levels 1-3 and now will only blow air if it is set to 4 or 5. now this is a big problem since we have been plowing about 37 hours straight just this past weekend during the blizzard we got. we cant keep the temp constant in the cab and its a big PITA! any ideas what could be wrong? i can easily access the unit through the dash and take it out.