Different set up obviously, but in my 4/4 I have an amplifier under the passenger seat connected to the speakers I installed behind the seats in the side panels. It connects via BT to my phone that sits on a mag mount between the seats.

I have a small remote button thing that connects to the my phone via BT that controls volume/play/pause and track forward/back. It's on the dash and within easy reach.

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"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10