take your dash apart, and make sure the ducting is completely connected. the pieces may have separated. and here's the trick to cooling down your rig as fast as possible:
roll down all the windows. turn on air to maximum setting, with the main vents on, and the fan on high. allow a couple of minutes for the hot air in the vehicle to be replaced by cooler air. roll up all the windows. switch the setting to bi-level. this will allow cool air under the front seats, cooling the rear of the truck much faster. and it's all the same air, so the sooner all of it is cool, the sooner you will be comfortable.
also, keep in mind that your air conditioner can only lower the temperature approximately 20 degrees. the sooner the hot air is replaced, the better.
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