// Make list of all harnesses.
// You can use both harnesses and cable units for report derivation. Its common base is DerivationSource class.
List<DerivationSource> sources = variant.GetAllWireHarnessInfos().Cast<DerivationSource>().ToList();

// You could add all cable units as well.
// sources.AddRange(variant.GetAllCableUnitInfos().Cast<DerivationSource>());

// Select an Aggregated BOM report type.
// We use "Name" property, because it does not depend on localization. "LocalizedName" does.
// To see see all the report type names, list all report types using Report.GetReportTypes().
ReportType reportType = Report.GetReportTypes().Where(n => n.Name == "AggregatedBOMReporter").FirstOrDefault();

// Create a new Aggregated BOM report from all harnesses in a variant.
Report report = variant.CreateReport("My new report name", sources, reportType);

// Save the project so the new report is saved into the project structure.
project.Save();