Buy a telemute lead and do it properly mate, you can then power the garmin from the car and mute the radio and amp the nav output to the front speakers.

$20 (£10) off ebay will get you one, you will just need to adapt the Garmin/telemute lead to splice then together (easy peasy mate).

Also the job is self contained, no hacking at wires and easy to un-install.

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.