I assume that you have the hot water tap open or you dont have one otherwise you would never get hot water to the heater core. Do a search on here for this one. It all has to do with where on your engine are you getting the water from. Some on here have moved the factory location for better hot water at idle. I think most of the discussion was around the LT1, but ICBW.
1996 Prostar 205 LT-1