In the annals of improbable rock classics, Janis Joplin’s “Mercedes Benz” looms even unlikelier than most. It’s sung a cappella, lasts less than two minutes, and wasn’t even written or recorded with a public release in mind. Yet, on the occasion of Joplin’s 70^th birthday, “Benz” remains one of the songs most associated with the late singer—partly because of the poignancy of its having been recorded on her final day in the studio, and partly because it’s one of the great light-hearted sing-alongs of the 20^th century.

“It’s a campfire song, isn’t it?” says Bob Neuwirth, who wrote the song with Joplin between sets two months before she died in 1970. “You don’t need any particular musical skill to sing it, and because it’s a cappella, everybody can tackle it in their own way. But I’m sure Janis would be shocked at the attention that that song has gotten over the years,” he laughs. “She’d just be shaking her head in disbelief at it.”

The endurance of “Mercedes Benz” has more to do with Joplin than the song itself, her co-writer modestly insists. “It is a marvelous glimpse of her personality,” Neuwirth says. “Of course there’s nothing ‘rock hit’ about it. It’s just the pure power of her charisma that causes it to be notable. The little cackle at the end—it’s so Janis, and that was more indicative of the woman than anything that’s been written about her. I mean, she was a funny girl. People love these Whitney Houston/Billie Holliday/Edith Piaf/tragic drama/poor-girl-went-wrong-and-died kinds of stories. It really wasn’t what happened.”

On the eve of her 70^th birthday, Neuwirth—a folk-scene legend who was one of Janis’ closest friends— talked with Yahoo! Music about how “Mercedes Benz” came to be recorded just three days before her death, as well as Joplin’s state of mind at the point of her untimely passing.

One thing he’d like to qualify: the idea that its being licensed for commercial use represents a sellout of the values established in the song. “When Mercedes used it in a commercial, people made a big thing about ‘anti-establishment Janis’ and ‘How do you think she would feel about her song being used in a commercial?’ And it really was nothing to make an issue of. It was meant to be a funny song, not an attack on the establishment, as other people have tried to establish. It wasn’t supposed to be ‘Those dirty motherfu---ers’-type stuff. It’s just a bar song!”

Quite literally, as he recounts the tale of its creation. On the night of August 8, 1970, Joplin had been booked into the Capitol Theatre in Port Chester, New York (which reopened in 2012, by the way, after a long renovation). She was in a foul mood because she didn’t like having been booked for two shows, and didn’t consider one of the opening acts, Seatrain, musically compatible. But Joplin was a huge fan of actress Geraldine Page, and Neuwirth knew Page’s husband, actor Rip Torn, because he had been enlisted to help Torn learn to play the guitar for his role in the movie Payday. So, as a surprise, he invited Page and Torn to show up at the restaurant next to the Chelsea Hotel, where Joplin was staying. “A bunch of Texans for cocktails meant a lot of margaritas and tequila. Then we all poured into this limousine and rode up to Port Chester,” about an hour outside of Manhattan. “Geraldine and Rip being there brightened her spirits considerably, but she was still in this un-serene state. She was doing her oppressed girl act about having to do two performances. So we repaired to this bar between shows.”

And in that fateful drinking session, “Mercedes Benz” was born. It originated with the title of a poem by beatnik favorite Michael McClure, “Oh Lord, Won’t You Buy Me a Mercedes Benz,” which Joplin had been singing over and over again as a singular refrain for a while. Alcohol helped spur it from a couple of lines into a complete song. “I actually have the words written down on a beer-stained napkin some place in one of my guitar cases,” says Neuwirth. “We were all sitting around banging beer mugs on the table and chanting what existed of the song, which was the first couple of lines—and we finished it at this bar. Geraldine and Janis fell in love with each other instantaneously—just bonded like they were sisters, real sisters—and the ebullience of the evening came through to that song. And then suddenly John Cooke, the son of Alastair, who was Janis’ road manager at the time, ran in and said, ‘Janis, goddam it, you’re on stage right now!’” Within the hour, Joplin was back on stage at the Capitol “and started singing this song—by herself, a cappella.”

On a bootleg of that show that emerged in the 21^st century, Joplin can be heard explaining that the song had just been written moments ago… and you can hear her slightly baffled band trying to join in on it, a little, by the time she wraps it up. But there was no accompaniment when she recorded it almost by accident during what turned out to be her final vocal recording session for the posthumously released Pearl album.

“In those days they’d record on big, fat, two-inch, 16-track tapes,” Neuwirth says, “but Paul Rothchild, the producer, would always have a little two-track, quarter-inch tape running in case somebody had a great musical idea and improvised something on the spot, so it would be captured and we could go back and reference it. What happened was, the tape machines broke down while she was recording, and there was a delay, and everybody was antsy, saying ‘Come on, hurry up.’ To sort of smooth everything over, Janis just sang that song, [joking] ‘Ah, have you heard my new hit?’ Everybody laughed, and it depressurized the situation. And it was captured on the two-track. Then,” he says, “she unfortunately made an exit.”

Neuwirth means The Big Exit, of course. “And the album wasn’t finished. They didn’t really have enough stuff. And Paul Rothchild thought it captured her personality, so they included it on Pearl. You know, it’s one of those songs that was never meant to see the light of day. It was just a bar improvise on the back of Michael McClure’s thought. You never know for sure what Paul would have done, but I doubt it would have made the album” if Joplin hadn’t suddenly passed away.

“And it became this song that I’ve heard people sing all over the world, man. I saw three Puerto Rican delivery boys singing it in the back room of a supermarket in New York once. Taj Mahal has a great version of it.” It’s also been covered by everyone from the metal band Jackyl to the classical pianist Glenn Gould. “It was meant to exist,” Neuwirth has had to conclude.

In a strange way, “Mercedes Benz” stands as Joplin’s last will and testament, having been put down so spontaneously on her last day of singing in the studio. (She did pop back into the studio the night she died, but only to witness the instrumental track being laid down for a song she never got around to singing.) How do we reconcile the tragedy of her death with the joy of this song?

“She made a mistake,” Neuwirth says. “She had an accident. She had been off drugs for a period of time”—although hardly off alcohol. Reports differ on how long or how severely Joplin had been using heroin again at the time of her death, but it had not seemed to have become a debilitating habit again. “And like any junkie will tell you, she used her usual amount of dope, but her body chemistry had changed enough that it was way too much. It wasn’t like ‘How much dope can I get into my body?’ It was her normal fix, but the body wasn’t acclimated to that level anymore. And that’s the tragedy.”

That it happened when Joplin did not seem to be at a particularly tragic point in her life?

“Absolutely not,” says Neuwirth. “She was really happy making that record—happy with the band and really looking forward to the next tour. And she had a cool guy going on. The more I think about it now, the more it brings me down.”

But hearing “Mercedes Benz” is hardly an unhappy experience for Neuwirth. “It just reminds me of the vitality that was Janis. I really thought she was on the way to becoming a great singer, as she evolved. You know, everybody’s clean and sober now, right? As clean as can be. And she would be a force to be reckoned with. Just the concept of Janis clean and sober is almost overwhelming. ”