The motor assembly is in two halves that rotate back and forth over each other as the motor racks push/pull the front half. The back half (shown upper here) fits over the front half (shown below here). This is a plastic-plastic bearing surface trapping and grinding dirt over the years. The more scratches and dirt the greater the friction when adjusting the mirror position. Clean all this up with a damp rag or swab (I used mineral spirits).