Did you give DMV a out of state title or MCO from the manufacturer? Did you ever have a CA title for the trailer? If a CA title was issued complete at REG 227 sections 1/3 if not then a REG 4017 side A only and pay the fee.
If you go to the DMV's website and look into a form HVTR 20 you will find the whole process laid out for you.
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