Remote Audio Power Wire

Simple questions:

2015 (same for 2012-2015 I think), Model III with nav and display audio (6") radio (non-JBL).

Which wire on connector L34(A) should be used as the remote audio turn on for an external amp?

Pin 8 - Power source of Antenna, Beige on the factory harness and blue on the Metra Connector.
Pin 9 - Amp power source (IG), not sure if this exists on the factory harness, but blue on Metra.
Pin 3 - Accessory (On), Gray wire on factory harness and Red on Metra
Pin 4 - Battery 12v+, Light Blue on factory harness and Yellow on Metra

I would assume Pin 3 or 8 would work equally well, but not sure if the antenna power (Pin 8) is only ON when the source is set to radio and turns off when the source is BT, USB or CD. If it is ON whenever the radio itself is powered, that would be optimal. This way, when the radio is off, so is the amp. However, this may be an issue for the nav voice and BT. Also, not even sure if there is any power to pin 9 at all.

The female Metra connector bridges Pins 9 and 8. Should this bridge wire be cut and both blue wires passed straight through?

The Amp I am using is a Pioneer GM-D1004 for the front and subwoofer outputs. The user manual states the power can be connected directly to the HU power if the fuse is 15amps or greater. I believe that Pin 4 should work for this, or am I always better to run the power direct from the battery?

This amp is small and will fit behind the lower glove box just to the right where there is a ground wire attached. I added a simple angle bracket mounted to the spot and the amp can easily strap to it with cable ties. I will build an under the seat 8" sealed box sub (about .38 cu ft) that hopefully will outperform all the premade ones. The sub box will be under the carpet with only the upper section that extends between the seat rails exposed, to look factory! I can share photos as the project progresses.

Thanks again for the help.