If you are asking where to buy it, it's a dealer part or you can find it at a mopar parts online site using the part number:
Clutch Starter Interlock Switch
56038959AB
If you don't know where it is located on your Dart or how it works:
06 - Clutch / SWITCH, Clutch Starter Interlock / Description
DESCRIPTION
The clutch pedal position switch is attached to the side of the brake/throttle/clutch pedal assembly. It is secured with an integral locking lever. A pin on the clutch pedal lever cycles the switch. A three pin electrical connector plugs into the switch.
06 - Clutch / SWITCH, Clutch Starter Interlock / Operation
OPERATION
The clutch pedal position switch is used to prevent starter motor engagement until the clutch pedal is fully depressed. An input from this switch is also used to either shut down or prevent operation of the speed control system when the clutch pedal is depressed. When the clutch pedal is depressed the switch contacts are closed and creates a complete circuit to allow the starter relay to energize the starter motor. When the clutch pedal is released, the switch contacts open and will not allow the starter relay to energize the starter motor.
If you are thinking yours isn't working, do the diagnosis below: