I had the same problem since I bought the car last October. I finally had it serviced last week and the problem is pretty much gone. Then again, I think I still smell it, but not nearly as much.

What was done to correct it is still a bit of a mystery to me. They hadn't completed the work order when I came to pick the car up and being they were quite busy, they told me to go ahead and leave and they'd mail the paperwork. As long as they weren't asking for money, I left. I haven't received the paperwork yet.

I'm not sure, but either the radiator or the heater core was replaced. I'll let you know when I find out.