I got the same problem and I just bought a new ypipe (including cat). There is a walkthrough somewhere on how replace the cabin air filter, I was gonna try that to fix it.

Also, check and make sure no liquids are leaking onto the exhaust and burning, that will make it up to your vents. good luck.

1995 Contour LX (ATX) KKM True Rev Sport Induction ( Video ) Alpine CDA-7894 / JBL 1200W AMP 12" MTX Subs 5x7 Alpine Mids Brullen mandrel bent quasi-dual exaust