some people have had problems with the smell of the spare tire in the trunk. Go to a tire store, and ask if you can have a plastic tire baggie. Put your spare in the bag, and see if that helps to reduce the problem.
If you are getting a sulfur smell, try changing gasoline stations. Some gasoline will have higher sulfur content than others, and if you can smell the sulfur, then your emissions system can't be very happy... If you live near a CA-emissions state line, try the CA-emissions gasoline, which will be lower sulfur than the national stuff.
have someone check the ductwork to the batteries. If the 12v was changed out, sometimes some mechanics do not replace the venting properly.
I agree with the other post - check your tires and wheel wells for compacted animal droppings or dead meat, and also clean the trunk to be sure that nothing is hiding under a carpet somewhere.
Also check the AC filter - that can easily get filled with leafy matter and insects and stuff, and could also be a source.
good luck!