the air conditioner in my 81 el camino comes out of defroster instead of vents. is it the vacuum hose and how can I find it

replace ALL the butyl black rubber hoses with Silicone as used on BMW's there are many colors available, from summitracing.com and most good auto suppliers have them too---was going to suggest a cigarette and puffin' it to find the leak, but you'd get a nicotine buzz and get addicted and the smoke would disappear before you could pinpoint the leak, so best replace ALL hoses with silicone..costs a little more but will not crack and leak~