CFR › Title 49 › Subtitle B › Chapter III › Subchapter B › Part 393 › Subpart G › Section 393.79
Windshield defrosting and defogging systems.
(a)Vehicles manufactured on or after December 25, 1968. Each bus, truck, and truck-tractor manufactured on or after December 25, 1968, must have a windshield defrosting and defogging system that meets the requirements of FMVSS No. 103 in effect on the date of manufacture.
Every vehicle I ever owned that had AC, did this. A friend of mine wrecked a late 70's land yacht because she couldn't be bothered/afford to get the AC fixed, and drove into the big cement structure for a lamp post in a parking lot on a humid night after work. In the UK, where AC was not common, we would carry a rag to wipe the inside of the windshield down periodically.