Congrats on completing the install. To answer question number one, there is no radio mute wire for your car.
There is a work around though.
Attempt at your own risk.
1. Purchase an automotive spdt relay, in-line act fuseholder, and 10 amp fuse (or whatever radio fuse rating is listed in the manual for your car) from Radioshack, Amazon or any automotive store. Amazon will be cheapest.
2. Remove your radio to gain access to the radio harness.
3. Wire colors are as folows:
Radio 12V - blu/wht
Radio Ground - brown
Radio Ignition - gray
Radio Illumination - green
Power Antenna - black
Left Front Speaker (pink+/purple-)
Right Front Speaker (lt. green+/blue-)
Left Rear Speaker (black+/yellow-)
Right Rear Speaker (red+/white-)
4. Use a DMM to measure the voltage on the radio ignition wire (gray wire). Should be around 12+ volts with ignition on, around 0 volts with ignition off.
5. Cut this wire. Try to turn on your radio. If it does not turn on, you cut the correct gray wire. If it does turn on, I don't know what wire you cut.
6. Connect the end of the wire coming from the car to the in-line fuse holder.
7. Wire the relay as follows:
87a - other end of in-line fuse holder
85 - 87a as well
86 - to a good length of wire that will be connected to the LI radio mute wire later.
30 - to gray cut wire going to the radio
8. Insert the 10 amp fuse in the fuse holder. Try to turn on your radio. It should function normally.
9. Strip back some insulation from the wire connected to terminal 86 on the relay and touch this wire to ground. The radio should turn off. Once you remove this wire from ground, the radio should power up and function normally. If this is all true, connect this wire to the LI radio mute wire.
10. Secure the relay in the dash behind the radio and re-install the
radio.
Basically what you have done is create a radio ignition interrupt. No different than turning your radio off and on with the key in the ignition cylinder.
Newer german cars are different as the radio's functionality is usually controlled via data... So I would likely figure out another way (radio mute wire/circuit or factory amplifier interrupt).
If you can install a Laser Interceptor correctly, you can complete the task above. I have absolutely no idea why you would want to do this though as the amplified speaker will be very loud.

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