He digs up a plutonium containing generator

Trick question - Mars is warm even at night due to the atmosphere being predominantly CO₂ ( - the greenhouse effect of/on the entire planet means that Mars is just as warm at night as it is during the day)

He uses the heater in his spacesuit (plugged into the Rover's power supply) 

He modifies the Rover's cockpit, creating a greenhouse like structure that traps the heat from the Sun

He uses additional solar panels to power the Rover's heaters