[AUSVX]

I've dissambled every single detail in my interior to bare metal.

The areas that separate the inside of the car to outside of the car are prone to being smelly are:
-the rear left vent on the inside of your bumper, maybe there's fumes coming in from there.
-On the firewall there really isn't anything that a leak could cause a smell to come in.
-there are those manufacturing holes on the floor that have gromets, maybe some are missing
-the fuel tank panel rear of the rear seats.
-Blown heater you'd have that shocking sweet coolant smell no matter if the car is on or off.
-leaking gearbox fluid is pretty potent too.

There are no vacuum lines inside the car that I know of.
the only lines in the car that I know of are brake, rigid heater pipes and heater of course and aircon rigid pipes and evaporator

When I floor it I get a little smell coming in too, but my main engine oil seals are leaking between the engine and gearbox, so that's my suspect.

Check my photobucket profile for images.

For aircraft you can buy this CO Carbon Monoxide sticker and it's yellow, CO makes the sticker colour change to red and indicate CO leaking into the cockpit, try and source one of them, I don't know how much they cost.