Looks like "Look Who's Talking," the No. 1 rental for most of two months on the Billboard chart, may not last another week on top. "Back to the Future Part II," which zoomed to No. 2 in just three weeks, is the strongest challenger "Look Who's Talking" has had during its reign. Normally, "The Little Mermaid" (No. 5) would have been the top rental by now, but, since it's priced for the sales market, sales are cutting into rentals. Woody Allen's "Crimes and Misdemeanors" has been a modest hit so far, moving up two notches to No. 16 after two weeks on the chart. Eddie Murphy's "Harlem Nights" slipped a notch to No. 8, the poorest rental showing by a Murphy movie since he became a star in the early '80s. Debuts: "The Bear" (No. 19) and "Old Gringo" (No. 25).