Had the same problem and my mechanic fixed it by flushing out the heater core. He had to do it three times with CLR and the crap that came out was amazing. heater blows hot air now to both sides.Ok how do I fix this problem. It only recently started blowing cold air out the driver side vents when the heater is on. Also can someone can tell me why the vents in my back seat barely have air flow.