Upgrading stock audio on my 2020 BRZ
 

It's amazing how many differences there are in multimedia systems of 2017+ US and EU BRZ models. I was sure there is stock amplifier in the trunk for door woofers based on videos I've seen. Not the case. At first I was thinking to add spare tire wubwoofer only, but after visiting music shop decided to upgrade the whole thing. Used stock multimedia head unit as I'm very happy with it.

Speakers: Focal PS 165 F3E 6-1/2" Expert Flax Evo 3-Way Component Speakers

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Subwoofer: custom made spare tire sealed cabin with Audison APS-8R 20cm Subwoofer

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Amplifier: 8 Channels Audison AP-8.9 Bit DSP

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Back (back door) speakers were removed, music scene setup in front. Music shop set each channel separately in amplifier's DSP, including sound delay for the sub.
Stock Focal crossovers were not used as all settings were made through amplifier DSP.

Very happy with install. Here is installation video made by the music shop.

https://youtu.be/SQa6pCgaEk4

nice! some days i wish i had a full size spare tire...

interesting use of sound dampening material in the sub box-- most shops in america just use polyfil/acoustastuf

It's common here for sealed custom cabinets. It's not for sound dampening per se but more to isolate possible air leakage form polyester part of the cabinet.