Definitely a high pressure area at the base of the windshield. That's how "cowl induction" worked. Basically, the pressurized air at the base of the windshield would be given someplace to go - into the engine bay. Properly done, the intakes on such vehicles were isolated from the rest of the engine bay to maximize the intake charge, but that's definitely how it worked.

That said, if the louvers on that vent panel are raised above flush and open toward the windshield - that is, they are not "scooping" toward the front of the truck - at higher speeds, the airflow across them would create a tiny low pressure area just behind each louver, effectively drawing heat out. A smaller, simpler version of what goes on with the RRE hood vent, popular in the DSM community.