Hello, i have a 1991 lebaron convertible, 3.0 v6, 146,855 miles. heat is not coming out of the heater ducts. it worked fine last night. it's coming up through the defrost. no matter what button i push it still comes out the defrost. when my mom had the car the defrost quit working. defrost works fine for me though, so did the other functions.(defrost, heat, vent etc.) i'm thinking the hvac unit itself might need to be replaced. maybe not sending the right signal to route properly? thanks for your help.