I have a 93 525 with about 160k. Every winter it seems as though I can smell gas in the cabin of the car when I start using the heater. I can not smell gas during the summer times when I am using the AC. I thought about a few things to explain this such as in the summer I typically have the recirculation on and in the winter I don't. Anyway, is this a common problem? Where should I start to look?

Thanks