Heater blows cold air--where is heater control flap or switch?

These cars are notorious for having blocked heater cores, causing the heater to blow cold air. That's because there is a heater core bypass hose near the firewall that allows the coolant to bypass the heater core. Eventually the heater core fills with gunk and no coolant passes through it. You'll need to disconnect the hoses from the heater core at the firewall and flush the core using a garden hose. Flush both ways till water runs clear. If you can't access the heater core fittings at the firewall, then follow the hoses from these fittings back to the engine and disconnect them at that point and flush from there. If you flush from the hoses in this manner, it is important that you clamp off the bypass hose at the firewall with a vice grip pliers.
Otherwise the flushing water will go through the bypass hose and not the heater core.

The first thing to look for is a low coolant level before suspecting any other parts.