Normally VW diesel (as well as many other) alternators have 3 wires, B+, D+, and W. B+ is battery positive, D+ is for the excitation circuit, and w for the tach. Connect B+ and D+ as they were before, with the remaining wire going to the remaining terminal. The terminals might be configured differently on this alt. You may have to crimp on a different style of connector. Post pics if you can.