[wwest]

The footwell heat/defrost airflow boosters will not run unless the engine compartment cabin heater blower is operating. Slightly lifting either of the 2 heat level control levers will start the rear blower via the switch connected to the 2 yellow wires. The Heat Controller module uses a reed relay setup to confirm that the rear blower is running, drawing the specified current flow. A low battery voltage can result in the rear blower not drawing enough current flow.