The television is still on as you begin to drift asleep on Saturday night. Suddenly, Richard Nixon's voice jerks you awake:

"There used to be a saying in America, "Would you buy a used car from this man?" he tells you, moving that awkward way he sometimes did during unrehearsed public appearances, his arms all akimbo. "Well, now you can. At San Clemente Dodge-Chrysler we're offering a full range of Chrysler Imperials . . ."

Live, from New York, it's Dan Aykroyd, doing his devastating Nixon impression. What comedian Chevy Chase did for Gerald Ford on NBC's "Saturday Night Live," Aykroyd does for Richard Nixon and Jimmy Carter. At the tender age of 26, with a hint of an accent from his homeland of Canada (he pronounces the word "out" as "oot," for example), Aykroyd is one wild and crazy presidential mimic.

"I love doing Nixon, he's so wonderful," says Aykroyd. "He's just such a character. He was harmful in his own way, but he's so sort of manipulative when he speaks; he has the ability to draw emotion out of an audience.

"I have a crack right down the middle of my nose just like he does, so I just grab an eyebrow pencil and pencil it."

To perfect his Carter act, Aykroyd watched the president's speech patterns during the evening news, noticing "the fact that he kept his mouth closed when he was speaking, except when smiling. And that he moistened his lips a lot and did his hair in a certain way. I started to work on the voice, then put on the jeans and the work shirt and did the first skit in a peanut warehouse."

And now it can be told: Aykroyd wears false teeth "to bring my upper lip out and help me with my speech." For his Nixon impersonation, he only uses "hair grease."

Footnote: Many "Saturday Night Live" performers expected the '77-'78 season to be the show's last, but everyone is back for another year. Meanwhile, Aykroyd has cut a blues album with fellow actor John Belushi. The two perform serious music under the name The Blues Brothers, and last summer they opened Steve Martin's act in Los Angeles for nights. Belushi (whose first movie, "Animal House," is an enormous hit) and Aykroyd are also appearing in Steven Spielberg's next movie, a comedy called "1941." Says 10th armor division ordnance sergeant, which is perfect; I get to move some tanks around, which I like a lot."