I have a 1990 Silverado (Ext Cab), V8, auto. It seems that I have heat, a/c & fan but all air is currently going through the defrost. When I push any one of the buttons (there are no knobs) to change from defrost to dash vents or floor nothing happens. I'm not highly skilled but can probably figure this out with a little help. I've seen a number of posts on similar problems but the responses were mostly "I have the same problem" and no concrete answers. So, is there anyone here that can offer some suggestions or fixes? Plus maybe someone could offer a web link to the locations of the components that may be at fault?