When a volume control is at the max the potentiometer is short cicuited. It is the smartphone volume wich control the system. It is the same as you want to instal a preamp on an integrated amp.
You have to put the volume of the amp at the max and it is the preamp controlling the volume. But you dont touch the volume knob in the car after .
We worked that bit out more by luck than design. How's the car going these days, you must have put some miles up on it by now, must admit best thing we did was get a CX.
