I've been doing battle with my ol' (1992) F150's lack of cabin heat/defrost. After replacing the t-stat, flushing the heater core, replacing controls because it didn't feel like the damper in the heat mixing box was moving freely... I took it to the dealer... and they find a small container of mechanical pencil leads lying in the mixing box and interfering with the damper. Oy! Moral to the story... be careful what you throw up on your dash... it might find it's way into places you don't want it to be.
