Locate and turn off the circuit breaker servicing the circuit you are installing your light heater unit on.
Remove the junction box cover or switch cover for the switch box. Remove the screws from the existing switch and pull it out, or remove wire nuts from the ends of the new cable. If you are replacing an existing switch, loosen the screw on the side of the switch and remove the two wires. Twist the black wire from the cable running to your circuit breaker box, around the top screw on the new switch, and tighten it with the screwdriver. Do the same with the black wire from the cable running to the light junction on the bottom screw.
Insert the new switch into the junction box and install the top and bottom mounting screws into the holes in the junction box. Tighten the screws with the screwdriver. Position a light switch cover over the switch and install the two mounting screws.
Remove the junction box cover for new installations, or the two mounting screws from original light fixture for replacement installation. Pull the light fixture down and remove the wire nuts from the wires connecting to the light fixture and separate the wires. Set the fixture aside. For new installations, remove the wire nuts from the ends of the three wires.
Match the wire leads from the back of the heater light combo, black to black, white to white, and green to green. Twist a wire nut onto each pair of wires, turning it clockwise to tighten them. Fit the base plate of the light heater combo to the face of the junction box. Install the two mounting screws through the base plate into the threaded holes in the junction box.
Restore the power and turn on the light switch. Pull the pull string hanging from your light heater combo to turn the heater on.