The supplemental heater on the TCH is called a PTC heater. Look here for a diagram showing it's location.
"The PTC heater is installed in the radiator in the heater unit and operates when cooling water temperature is low and normal heater effectiveness is insufficient.
The air conditioning amplifier switches the circuit in the PTC relay and operates the PTC heater when the operating conditions (cooling water temperature is below 55°C (131°F), setting temperature is MAX. HOT, air outlet damper position is FOOT or FOOT/DEF, and blower switch is not OFF) are met."
I normally just set the temp and fan, so I can't say how noticeable the extra heat of the PTC heater is.