The heater inlet hose on a 1992 GMC Sierra 5.7L is located on the
engine's water pump. It's a hose that comes *from* the water pump and goes *to* the heater core. It'll be a relatively thick hose, likely a top hose connection on the water pump. Because of the age and potential for variations between specific models, looking up a diagram specific to your truck's year and trim level (using a parts website like AutoZone or Advance Auto Parts and entering your VIN) would be highly recommended to get the exact location.
Keep in mind: It's often easier to see the *outlet* hose (going *from* the heater core *to* the engine) first, because it's usually routed more visibly. Once you locate the outlet, trace it back to find the inlet.
