As you know, in the 1996 model year, rear heater vents are located on the left wall, close to floor. So, you say that these vents are blowing cold air in your car. Correct? Are you getting heat in the front or not? If no heat in the front either, then one possibility is that the thermostat is stuck in closed position. If there is heat in the front, then I think hot coolant is not reaching the rear heater core or somebody has disconnecetd the line that takes coolant to the rear heater core; some people would do this if the rear heater core develops a leak. If the coolant line has been detoured to bypass the rear core, you should be able to see this under the car. | 
