at what temp should you start plugging in your d-max. the cel came on and the diealer said it was because the plug in heater cord was bad. the new cord doesnt seem to be working but the manual and the cord say it will not work until temp gets below 0 degrees f. any info on this will greatly be appeciated. if you plug truck in on cold night... can anyone tell me what average temp should be at start up??