I do not know how that generation of Caprice is designed; maybe someone else can help out.
On my Caprice I have a similar hose that goes from the Heater Control Valve all the way under the dash. It looks similar to this one:
Perhaps this has gone bad on your caprice? They are fairly cheap -- this is an image from Autozone.com and it was at 18$ US Dollars.
If it is blowing heat though as you say (only venting and not switching to defog and etc) then I would think most likely it has to do with the vacuum control switch/mechanism or a broken hose (if your caprice is designed in that way like mine is).