Power output needed to go through a car
22 dBm is a pretty significant signal, which should be plenty at such close range.  However, as Dave Tweed said, it's not going to go thru a metal roof or metal hood at all.  You need to make sure this PCB has some kind of visibility to the roof of the tunnel, while considering any extended metal surface to be opaque.  Your 22 dBm signal is strong enough that some will also bounce off the tunnel walls and come in sideways thru the windows of the car.

Still, you shouldn't expect reliable reception in a mostly enclosed metal space, like under the hood or in the trunk.  A good place to put the receiving PCB would be on the dashboard or under the plastic of the dashboard.  That way it will catch the transmitted signal directly thru the windshield.  Another good place might be under the rear window if the car is a sedan, for the same reason that under the windshield will work.