One of the rear electric window mechanisms on my '84 300SD is broken. The motor and arms are intact, the only problem is the white plastic slider that fits into the track attached to the bottom of the glass. This plastic piece appears to be riveted to the arm. What is the best option for fixing it? (I tried epoxying the plastic piece back together but it wouldn't hold.) I would like to replace as little--read as inexpensively--as possible. Thanks