can you provide some pictures? I am wanting to add a sub and amp to my 2019 mazda 3 with bose audio.So i just did this install, not too bad, first i decided to tap into the rear door speakers to a LOC2SL, found out that the rear speakers dont handle any bass... so i had to tap into the built in Sub, located under the diver kickpanel, there is two wires going to it, one Blue and One Red. i found out that you could only tap into the Red one, the blue one didnt provide any signal for RCA, for the 12v power i ran a 0 gauge from the battery to the amp, but for the remote 12v i found that under the passenger seat there is a remote 12v there that turns on and off with the car. it was a yellow wire.
it works good. also i have a 2019 Mazda 3 sport premium package.
if you want i can provide pictures.