Bob Dylan is popping up in the most unlikely places. First he coauthors ''The Rest Ain't Important'' with pop songwriter Glenn Ballard, slated to be cut by Was (Not Was) vocalists Sweet Pea Atkinson and Sir Harry Bowens. Then Dylan's most recent producers, who happen to be David and Don Was, write a new song with Dylan, ''Shirley Temple Doesn't Live Here Anymore,'' intended for Paula Abdul. (Abdul ended up passing on it.) Dylan then lends his pipes to Brian Wilson's upcoming album, joining a backup chorus that includes Abdul, Belinda Carlisle, and Glen Campbell. Are you still with us? Well, Dylan has also just collaborated with Michael Bolton on a tune for the follow-up to Bolton's 1988 multiplatinum Soul Provider. Written in two days at Dylan's California home, ''Steel Bars'' will appear on Bolton's new LP Time, Love & Tenderness. Bolton reports that the pop bard, who called him out of the blue, was an enthusiastic partner. ''He just kept coming up with all these great lines,'' he says. Still to come: Dylan teaming with Madonna for ''Just Like a Virgin.''