Whenever I turn on my defroster and I'm at a stoplight it makes my car shake but if I have my heater or A/C on it doesn't make my car shake. Anyone know why?

 

My car "vibrates" at idle when I have the fan on high because the altenator is placing a load on the engine trying to produce all the electric current it needs. I don't worry about it as older cars will have a less smooth idle with all the wear and tear.