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I have a new AM-FM radio headunit to install in my console and have 2 questions about wiring power and location.

Instructions have 2 power leads.
1. Battery
2. Ignition

My understanding of what these leads do:
1. Main power supply to headunit, when 2. is a closed circuit and a constant lo-amp power supply to maintain sta., time, etc memory [when 2. is open circuit or off].
2. Closes a circuit to draw power from 1.

Q. 1: Can the 'COM' terminal of my battery selector be used for 2.[Ignition]?

Q. 2: Will radio audio receive TACH interference if placed too close to TACH [Yamaha F70 analog TACH]?

Thanks
Joe