The company for all your harnesses is Kabeldavico.
Frank David Kozlowski is behind this company and he is a very helpfull and honest guy. He is highly recommended
He has sets for
a) Audio20 to NTG2.5
b) Audio50/comand to NTG2.5
His product is of high quality reasonably priced, so it is not worth trying to DIY. For SEA region, you may contact me if you wish to buy.