Fitting a rear window heater

Clear vision both in front and behind is essential for safe driving; but while most cars have all the necessary equipment for keeping the windscreen clear, similar equipment for clearing the rear window sometimes has to be fitted as an extra. A heating panel fitted to the rear-window and wired into the car’s electrical system will keep the window free from mist and remove ice and snow.

The heating element is mounted on tacky paper and protected by a paper backing sheet; it is usually supplied with wire, terminal tags and an on/off switch. If the switch does not incorporate a light. buy a separate warning light. This will help you to remember the heater is on; if you leave it on for any length of time while the car is stationary, it may drain the battery. When buying a window heater. make sure it has enough power to do the job; one rated at about 80 watts should be sufficient.

Mounting the heater

Careful positioning of the heater is essential to ensure the right area of the window is cleared. Make a paper template of the heater and fix this centrally on the outside of the window with adhesive tape. Sit in the driving seat to check the positioning is correct and adjust it if necessary. Wash the inside of the window with detergent and rub over the area with a clean rag dampened in methylated spirit to ensure the window is completely free from grease. Place the heater on a flat surface and remove the backing paper: cut the paper in half vertically and reposition it on the heater, leaving 50mm between the two pieces. Line up the heater on the inside of the window with the template on the outside and press it firmly against the glass. Carefully remove the backing paper, one piece at a time.

Wiring up

Ideally the electrical supply for a rear screen heater should be taken from the accessory terminal on the ignition switch to make certain that when the ignition is off, the heater is also off; alternatively the supply can be taken from the outlet side of the fuse box. If there is no suitable spare fuse in your car. fit a line fuse in the supply lead to the heater.

Mount the operating switch in an accessible position on the dashboard. Fit a terminal tag to the supply lead to the heater and attach this to one of the switch terminals. Fit a second wire to the other switch terminal and connect this to the ignition switch or fuse box.

Plan carefully where the supply lead for the heater will run in the car, bearing in mind that for a high wattage accessory like this, you should keep the wire as short as possible. The wire can be run behind panels and along headlining seams: it can also run beneath the floor covering, but keep it away from normal wear areas to avoid damage. If the wire passes through a hole in a metal panel, line the hole with a rubber grommet to protect the wire.

Run the supply wire from the operating switch to the heater panel, securing it with PVC insulating tape. Fit a terminal tag to the free end and connect this to one of the terminals on the heater. Fit terminal lags to a second wire, connect one end to the other terminal on the heater and the free end to a suitable earthing point. Warning light If you are fitting a separate warning light, mount it in the fascia next to the operating switch. Connect one end of a length of wire to one terminal on the operating switch and the other end to the ignition switch or fuse box. Connect two wires to the second switch terminal, taking one to the heater and the other to one terminal on the warning light. To complete the circuit. fit a lead to the second terminal on the warning light and take this to earth.

Finishing off

Check the wiring is secure, switch on the heater and. as it warms up. smooth the heater down with your hand. Turn off the supply and leave the heater to cool down, check it is firmly fixed and slowly peel off the tacky paper. Leave the installation for a few days before washing the inside of the window; this allows time for the adhesive to settle down. Warning Always be careful when cleaning the inside of the screen: don’t rub too vigorously and be particularly careful to prevent sharp objects rubbing against the heating element.

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