Audi Half Amplified Harness adaptor. PC9-401 Audi Rear Amplified ISO Adaptor. A harness adaptor lead allows you to install a car radio of your choice with out cutting any of the existing car wiring. All harness adaptor Leads are ISO ended and will plug directly into most models of car radio. To activate rear speakers, the blue & White wire on the Red mini ISO plug also needs to be connected to the Blue wire coming off of the New Head Unit (marked Amp Remote). At the same time the blue wire on the PC5-90 adaptor also needs connecting to the Blue wire off the New Head Unit. Use this harness where you have four wires in the Brown ISO connector.