My 83 300SD door locks work only intermittently. For over a year, when locking or unlocking from the drivers side door with key, many key turns have been required to make the locks work. When I manuever the central locking motor in the trunk, it'll work better, but not for long. The motor clicks when the key is turned, it just doesn't always spin. Is this a common failure with this motor? Can a new motor be located easily (it's not in your part catalog)? Were these same motors used on all 126 sedans? Any other ideas?