Lucky for you there was a dicussion on this recently. Check out this thread: No Heat
Basically check to see if the heater core is getting hot by feeling the heater hoses, they both should be getting hot. If they are then check to see if the blend door in the heater box is moving correctly. I believe yours is vacuum operated and located under the hood near where the heater hoses go into the firewall. If everything checks OK then backflush your heater core and recheck the system. Then check back with us.