Installing a mirror defogger will stop your bathroom mirror from fogging up after a hot shower or bath.

Preparation
Remove the bathroom mirror very carefully. Have a friend help you if it is a large mirror. Place the mirror on a very soft, flat surface so it doesn't break.

Basic Materials

• Tape measure
• Drill
• Electrical box
• Wire cutters

Step-by-Step

1. Where the mirror was on the wall, measure from the counter to eye level - this is height where the defogger will be placed.
2. Measure side to side, find the center. Mark it.
3. Take these measurements to the mirror.
4. Peel off the backing of the defogger - it's very sticky.
5. Carefully place the defogger onto the backside of the mirror. Work from the center. Do not put pressure on the mirror - it will break it very easily if you push down on it or put any weight on it.
6. Apply the defogger slowly and evenly and smooth out any bubbles. It doesn’t have to be exactly square.
7. After the defogger has been applied, measure up from the bottom and side to exactly where the terminal is on it because you are going to drill into the wallboard to make a hole for it.
8. Mark the measurements on the wall and drill a hole into the drywall about 2" x 2". Don’t want to apply too much pressure because you are not sure what’s behind the wall.
9. Now disassemble take your light fixture down. Here are probably just a couple of Phillips head screws holding it in place. MAKE SURE YOU TURN THE POWER OFF AT THE BREAKER BOX BEFORE DOING ANY ELECTRICAL WORK.
10. Now go underneath the cabinet and drill a hole for the transformer. You aren't going to put it in the wall, but inside the cabinet. Be careful drilling so that you don't hit any plumbing.
11. Insert the wire and drop it down through the wall to hook up with the transformer beneath the cabinet. It might take a few tries to get it where it needs to go. You can use a clothes hanger to fish the wire out of the wall. It might take a few tries.
12. We used ordinary Romex wiring, regular 14/2 gauge you can buy it anywhere, very common, and it’s not expensive.
13. Pull the wire into your electrical box where you are going to make all our connections. Once you get the wire in the transformer box, strip away the insulation around each wire to make the connection.
14. The low voltage transformer will set on top of the box. This carries 110 volts, but you only need 24. Stick the transformer on top of the box and to meet code.  Inside the box, match up black wire to black, white to white, and wrap the ground wire around the screw hole of the box.
• In your light fixture, connect the heater wires to the fixture.
• Plug in the wire from the heater and it’ll run down the wall into the transformer.
• Replace the mirror and try to put this back into the wall at the same time.
15. Get the wire through down to our transformer. We need to make our hole here so big so our connectors can go through and not hold the mirror out. Fish the wire out. Mount it on the transformer and you're almost done. Mount the box and put the cover on it. The transformer box will fit right along the back wall of your cabinet.