The air conditioning condenser fan is designed to pull air in from the rear of the car and recirculate it back out to the rear. The truck insert is designed to allow the air to flow in that direction. With the trunk liner out, that fan is going to be working against all of the air pressure that is developing from under the car. You could always just test the car with the trunk liner inserted and with it removed and see which method works best. I have wondered what would happen if someone was to install a pusher fan on the condenser and then drove the car without the trunk liner. It would seem that you would be able to increase the air flow, but I doubt that it would work very well with the trunk liner installed. I have no way of testing that theory in my Pantera because the condenser is in front of the radiator. That modification seems to work very well in 100 degree Arizona weather.
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