A block heater cord extender features a frame for mounting to a vehicle, a female electrical socket carried on the frame for receiving a male plug of the vehicle's block heater cord, and an extendable/retractable arm having a proximal end movably mounted to the frame. The arm is extendable and retractable in length, and carries an electrical plug at its distal end for mating with a mains power electrical outlet. Circuitry of the extender includes a wireless transmitter arranged activated by the mains power supply when the extender is plugged in. The transmitter sends a signal to a separate warning unit placed inside the vehicle, which reminds a vehicle operator to unplug the extender before driving away.