This just happened to my friends car and my lawn tractor not so long ago. The rotten egg smell is the battery. I guess that can happen when its been frequently overcharged or when they are old and cannot support the charge. So i'd start with your alternator and battery. Both of those fixed my friends toyota and a new battery fixed the tractor the reason why the tractor battery got like that is because it was old and sat on the charger all winter. I highly recommend not parking your car in an attached garage. My house smelt for days. Hope this helps