Car Parking used by someone else

I purchased an apartment in resale with car parking number mentioned in registration (in original conveyance deed also). When I went there, found same car parking is used by someone else. He was given a letter by the builder 2 years back to use that parking slot (few days earlier than the original conveyance deed of the flat I bought). Should go legal way to have the car parking slot in my registry, against whom - seller or Builder? Or take other slot from builder costing extra registration fe