1985 Nissan 300ZX Just blowing hot (engine) air through def

I need to learn as much on why my air conditioning is blowing hot (engine) air up through my defroster vents under the windshield. The blower motor seems to blow, only not through any of the routes the regualr buttons designate, i.E. Face, feet, face and feet, etc. Of course, I should add that it started to just blow cold air (through the normal routes) only when you'd let up on the gas. That is to say, it would blow really hot (engine) air through the designated routing when you were on the gas. I have very little money and am afraid of beng ripped off by the many unscrupulous mechanics I've encountered over the years. I have tools and an aptitude for mechanical work. If I know the solution, I can certainly give it a go, and go to a pro if/when I find it too daunting.