Lol, coax/cable impedance is not a big issue at audio frequencies!
So, what exactly is the setup and issue?
What I want to have is both Vauxhall Telematics and Nokia Bluetooth kit connected to back of stereo.
Currently, I have:
Telematics audio connected to the audio connection to Nokia CK-7W kit, leaving Nokia kit unplugged (having both connected like that is probably bad idea!).
Telematics is complaining of Audio Line Out, and the volume of phone calls is very low. I suspect this is because I have a low resistance across the audio looking towards HU from telematics. I am guessing this is because telephone prewired cars have an audio transformer/isolator fitted somewhere (U29)?
As a test, I have put wires from telematics straight onto back of HU, and volume fine, but get a lot of 'cellular type' interference on it, presumably due to using wrong cable type.
So, I guess I need to rerun audio from telematics in boot to HU in a decent screened cable?
In order to be able to use the Nokia bluetooth kit as well, I was going to but in a DPDT relay, activated by Nokia mute line, to switch the source, but unsure if it has the ability to sink the current to drive relay. Despite the wiring diags, the telematics does not need mute line, must happen over CAN.