Carbon Monoxide

NOT normal. I don't care how bad (burning oil, whatever) the junk out the back "is" you should not be getting any in the car

Check the entire pass. compartment carefully for leaks and holes. Don't forget "underneath," and rust holes in the quarters, fenders, etc,

If it is "that bad" you might consider stripping the seats and carpet so you can really look over the floor. And, take a GOOD look (like from inside the trunk) at the seal

Consider using two people, flood lights "shining through" or even water spray

I CERTAINLY would consider some sort of detection for CO until you get this ironed out.

Depending on where you live and the situation with local / nearby fire depts you might even get them to sniff it for you. Those guys, generally, are concerned with safety