Why would the AC work and the heat not work on a 1990 Toyota pickup?

If the AC works but the heat does not in a 1990 Toyota pickup, it could be due to a few reasons. One common issue is a malfunctioning thermostat that might be stuck open, preventing the engine from reaching the necessary temperature to produce heat. Another possibility is a problem with the heater core, such as clogs or leaks, which can impede the flow of hot coolant. Lastly, there could be issues with the climate control system or controls that regulate the heat settings.