If there is a lesson to be learned from "You Light Up My Life" - and there certainly is - it's never to gain- say the commercial potential of high-powered, shameless corn.

Currently in the No.2 spot in Variety's top-grossing film chart and theaters, "Light" is the handiwork of Joseph Brooks, who wrote, produced and directed the film, composed, arranged and conducted the music, and probably e ven loaded the cameras as well.

Brooks is the latest in a series of clean-cut, commercially shrewd young men - Tom Laughin of the Billy Jack films and Benji's benefactor Joe Camp are two previous examples - who have come out of Middle America determined to give the public what it wants, no matter what.

Brook's background is in commercials. He's won 21 Clios, written the music for blockbusters like "You Got a Lot to Live and Pepsi's Got a Lot To Give," and at one time calculated that, for what it's worth, more people in the United States listened to his music than any other composer in the world.

In this first feature film, Brooks has chosen to tell the story of a young girl, an inspiring Los Angeles singer-songwriter, fighting to express herself. She has to fight her father, a well-meaning old codger who wants her to be a comedienne. She has to fight her intended, who wants her to be boring. She has to fight herself, for falling in love with a steely-eyed film director who picks her up one night at a pay phone. (Yes, a pay phone.)

At the end, after all that fighting, she discovers, surprise, that she is Her Own Best Friend. "I learned that I gotta depend on myself," she tells Pop, fighting back the tears, "but that's okay because I'm really good person to depend on." With that she's off to New York and (what else but) instant stardom.

It is probably not fair being too cynical about "You Light up My Life"; It means well and has a certain, hokey, amateurish charm. But Brooks is so relentless, so shamelessly intent on pulling out emotional stops most people have fotgotten even existed, that it is hard to take his film with total equanimity.

Both the good and the bad are illustrated in the choice of Didi Conn, since seen on TV's "The Practice" and "Sugar Time," to play the lead role.

Conn is of the spunky, winsome, archetypically gamine school of actresses, a we-try-harder combination of Woody Allen and Barbra Streisand. A little vulnerability is always nice, but Conn is so ferociously, overwhelmingly vulnerable that one is often afraid to look at the screen out of sheer terror at what awful thing may be about to happen to her.

Please, Joe Brooks, next time give us a break. Let up on us. Even a little bit. It would be appreciated.