Automapper expression must resolve to top-level member

You are using :

 Mapper.CreateMap<Source, Destination>()
 .ForMember( dest => dest.OutputData.Cars, 
             input => input.MapFrom(i => i.Cars)); 

This won’t work because you are using 2 level in the dest lambda.

With Automapper, you can only map to 1 level. To fix the problem you need to use a single level :

 Mapper.CreateMap<Source, Destination>()
 .ForMember( dest => dest.OutputData, 
             input => input.MapFrom(i => new OutputData{Cars=i.Cars})); 

This way, you can set your cars to the destination.