DML FAQ : Body : My HVAC doesn't come through the vents I've selected.
This is a vacuum problem. All of the little devices that direct the air to the right opening are operated using vacuum that comes from the HVAC control panel. The hvac panel gets vacuum from the engine by way of a vacuum line that runs though the firewall. It is easy to fix but very time consuming.
A FSM has some helpful tips, but you can do it without it using very methodical methods to find the leak. I'd start by checking at the engine side and work towards the HVAC control panel.