Your steering wheel is off gassing. The smelly plastic is CAB=Cellulose Acetate Butyrate off gassing as it deteriorates. Butyric acid being off gassed is also found in vomit etc. CAB is a malignant plastic and the gasses (butyric etc.)being emitted can damage metals and other materials in proximity. Best to replace wheel as the plastic is deteriorating and will eventually fall apart which is a safety concern.
http://cool.conservation-us.org/waac/wn/wn24/wn24-1/wn24-102.html
http://dwarmstr.blogspot.com/2013/02/why-toolboxes-and-tool-handles-stink.html