1992 Chevy Lumina Help PLease

Engine problem
1992 Chevy Lumina ? Miles

I have a 92 Lumina and I have recently replaced my radiator and my water pump due to overheating problems. I am no longer having overheating problems but I am not getting any heat in my car. It blows out but it's cold air. Someone please help me in telling me what it could be, I have exhausted my money replacing everything and now I am driving back and forth to work with no heat or defrost! AHHH! I had posted this originally and forgot to add this, my dad had replaced the radiator for me and upon doing so found that one of the fans had been disconnected. He reconnected it and after the water pump was fixed again I was still getting no heat, he thought that maybe the thermostat was bad and that the fan was keeping the engine too cool so he disconnected the fan until this weekend until we could look further into it, he also put a piece of cardboard on the radiator to help the heat to come into the car. On the highway I was getting GREAT heat and then after I was going slower off the highway it went back to cold, any other suggestions?