I'm getting carbon monoxide entering the D90 passenger cabin. This is a 300 Tdi that I've had for a couple of months. It was occurring with the hard top on and it's the same now that the soft top is installed. The exhaust system is new and I've had it checked for leaks by two different muffler shops. I discovered a significant improvement by closing off the heater vents with duct tape. This would lead me to think there are two issues: there's some kind of CO blow-by happening under the hood, and the heater is pulling air from the engine bay instead of fresh air from the outside. Any ideas for next steps in troubleshooting?