[Blackbirdws6]

Install scoop where trap door would go!

The above is partially serious but you really need to get a wind speed meter to test different spots within the interior to see where the "high pressure" zones are. Otherwise you are punching a hole somewhere and it may not be in the right spot. I can likely borrow one from work if you want to do some half-ass engineering sometime. I'd think the rear deck before you get into the rear hatch/trunk area would like be a good spot. A scoop or two installed there with an exit in the bottom of the trunk would provide a reasonable path for air while avoiding issues of bringing exhaust fumes into the cabin. If you are very concerned, install a flap with a spring at the exit. When sitting still, exit is closed, at speed with some air flow/pressure, it's open. You'd have to figure out the right spring tension of course.