Mercedes-Benz of North America will give $750,000 over a three-year period to underwrite the Celebrity Series concerts of the Los Angeles Philharmonic, it was announced Wednesday. In 1988-89, the Mercedes-Benz funds will be used to underwrite 14 Music Center concerts, including an April 7, 1989, appearance by the Munich Philharmonic (on its first U.S. tour), conducted by Sergiu Celibidache in his Los Angeles debut. Mercedes-Benz is the second automobile company with which the Philharmonic Assn. has an ongoing relationship. In the past, Isuzu Motors Ltd. has sponsored domestic tours by the orchestra.