[Christ]

Change the location of the air intake through the cabin air filter to a place under the car, preferably in front of the engine compartment so you're not getting heat-soaked air.

Basically, the air under the car is supposedly the coldest air there is, and where your cabin intake is right now, you're getting heat-soaked air under pressure (it's probably at the windshield cowl).

If you're getting cooler air initially, this makes it so that you can just turn the blower on, remove access to the heater core (turn the selector to cold) and that air (especially at highway speeds) would be sufficient to cool you.