Alec Baldwin was on the Santa Monica Freeway with a plastic quart cup filled with chardonnay and ice between his legs.

“I’m driving down the road, I’m having a drink,” Baldwin recalled of that day in 1983. “It’s 4 o’clock; I’m supposed to have a drink. But one day I went, ‘I don’t see any body else in their car with a plastic take out container filled with ice and wine. They’re drinking coffee, they’re drinking Diet Coke. They’re not drinking wine.’ ”

Baldwin writes of his booze-soaked, drug- addled 20s, and the various self-revelations that finally led him to clean up.

The “30 Rock” star describes the worst times of his addiction as the ” ‘Sixth Sense’ phase” – “when you’re dead but you don’t know it.”

He’d find himself in places where “sex, drugs, booze” flowed freely.

“The room was full of people getting high and drinking and talking about all this bulls- – -. All those 3 a.m. conversations where everybody’s talking, urgently talking about something that doesn’t really matter,” he writes in “Moments of Clarity,” a new book of personal testimonies of addiction compiled by actor Christopher Kennedy Lawford.

Baldwin would end the night at an arcade warehouse playing “Galaga.”

“I would play video games from, like, 9 a.m. to 11 a.m., and I would wind down. Then I’d go home and go to bed,” Baldwin writes.

“This was the only way I could go ‘beta’ and go into that state I needed to be, where I could calm down and take my mind off everything. I didn’t want to see anybody, talk to anybody, deal with anybody.”

A “moment of clarity” came when he saw pity in the face of Julian, the person who ran the parlor.

“I was doing a show then [‘Knots Landing’], making tens of thousands of dollars a week, which was part of the problem,” he writes.

“Julian would put the key in the lock and open the door, and he would just kind of look at me like, ‘Wow, I’m glad I’m not you.’ ”

Baldwin agreed. “You got no idea, Julian. Julian, I need you. I need you to get that key and open the f- – -ing door and let me in. I got to play ‘Galaga.’ ”

Then there was the disapproval in the eyes of his girlfriend’s grandfather, who noticed how many times Baldwin took bathroom breaks.

“He’s looking at me with the unmistakable look, like, ‘There’s just something not right with you.’ Sweat running down me like I had gone through a triathlon.”

Six weeks before his 27th birthday, he joined a support group.

“God got me sober. That day, God was a black, 65-year-old retired postal worker named Lenny,” Baldwin writes. “Lenny said, ‘You never have to feel this way again if you don’t want to.’ ” scahalan@nypost.com