Just curious. On a 2nd gen F Body, there is a large foam seal in the main ducting for the dash. The foam can disintegrate, and air can leak around the opening, lowering the intensity of the air through the vents. Also, the main duct under the glove box, which this seal is attached to, is a known weak spot, and can crack, leaking more air.
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1976 LeMans B09 Freeway Enforcer, 455/M40 Smokey
1977 Trans Am, 400/M21 Black/Gold Bandit. 44K actual miles
2017 Sierra SLT 1500 Z71 4X4 
2019 Canyon SLT Crew 4X4 
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