Stainky inside -- but my exhaust system is fine. How to fix?

[FinnJohn]

Hi guys, I have a domestic problem I'm hoping to get advice on. My Benz gets diesel fumes in the back seat. On a recent trip back from the beach, it nearly sparked a domestic revolt. Seemed like the more the windows were open, the stinkier it got. I've put a band-aid on the problem by burning biodiesel, but I'd like to fix it.

The exhaust system is watertight, though. I put my hand over it and the pressure built right up. So somehow the fumes are being sucked in.

Would new trunk-lid seals help, do you think? Or do I need to look at where in the slipstream the tailpipe vents?

Thanks!

--Finn