Certainly use relays. Also an alternator, as they take a lot of current
Onne van der S. MMOCno 60520 Moderator
2dr 1971 White DAF 55 (with hopefully a 1600cc engine soon)
2dr 1973 Bergina (DAF 44)
2dr Estate 1975 DAF 46 in red
2dr saloon 1972 DAF 44 in Mimosa
The rear screen, saloon or traveller draws around 8 - 10 amps and should be used for up to ten minutes. The front screen version that was out a couple of years ago draws 13 amps. The latest (available from the factory since summer 2006, but dealers might have older stock) has a wider heated area and so draws about 0.5 amps more. The front screen is a quick heat and shuold not be used for more than 3 or 4 minutes - but in my experience, 1 or 2 minutes is more than enough.
It is possible to get 12 volt switches that will easily handle 15 amps (eg the headlight switch) but any automotive "accessory" relay from any car parts shop will be suitable - 20 amp, 30 amp, etc. Look for 4 terminals: 1 from the switch, 1 to earth plus a supply and load pair. The relay will come with a wiring diagram.
Basically, for each circuit take a wire from the unfused and unswitched side of the car wiring harnbess through a new fuse (15 amp) and then to the relay supply. From there to one terminal on the screen. The other screen terminal goes to earth. Traveller rear screens should be wired in series. The supply to the switch is taken from the ignition switched ciruit. From the switch to the relay and the last relay rerminal goes to earth.
In operation, when turning on the ignition you supply power to the heated screen switch. When operating the switch you supply power to the relay. This then supplies power to the screen.
Good luck, you'll need them this coming week if the weather forecast is correct.
