Diagram for 84' Chevy Scottsdale?

For some reason the defrost in my pickups heating system doesn't work. Everything else does but not the defrost...winter is coming up and it's important to have defrost in my vehicle. I was wondering if anyone knows where I can find a diagram for the vacuum lines or fuses for the heater in an 84' Chevy Scottsdale? I have a book for my truck...but says nothing about either of those. Or does someone maybe have an idea as to why it's doing this? Any input would be very much appreciated - - - - Thank you!!