The Racor filter has a built-in thermostat that I believe is set to operate when the fuel is below 40 degrees. You can hook one wire directly to any "switched 12V" source (there are several places in the under hood fuse box I believe) so that when the key is on power is supplied to the filter. The other wire to a ground. The heater draws 20 amps so a 30 amp fuse should be in the line. Can use a relay but not really required.