1995 Altima - No Heat

OK, I've got a '95 Altima that blows out absolutely no heat. I've replaced the thermostat and made sure that the radiator is full. The blower motor works great. In fact, all it does is blow out cold air. So what am I missing? I thought that the only thing you needed for hot air is a working blower motor, thermostat and plenty of coolant. Obviously, I'm wrong.

I have a 93 w/ the same problem. In the center of the firewall (or very close to it) there is a lever that you can manuever a bit. This controls the heat. Have your car running and play around with it, the heat should work. Mine works for about a week, and then the lever goes back to its normal position (but its a quick fix).