Ok here is my idea, so pop to Audi to get correct fixings for your Audi seats. Weld in, put in new caddy life carpet which I would have in black with sound deadening under it to reduce road noise.
Ceiling I would also do black with 4,6,8 LEDs your choice with 4 tweeters 1 in each corner.
The sides I would do a different colour charcoal or grey so you get that two tone affect I like that I would make the window panels flush, like mine 1 full panel.
I would get a mould made or little boxing for speakers at each B pillar and and 2 at the rear corner. Drivers side build in 2 IPad charging docks into the panel with usb charging two. So you can use it and still charge.
The tailgate maybe put in the Kicker slim sub I don’t know the panel on the back of these tailgates but it looks like there is some space in there so could you hide the amp in there to. If it don’t fit there then put it in the Lower wheel arch panel. The material would be your choice due to budget if you can afford alcantara at £55per meter or carpet at £5,6,7 per meter that’s your choice.
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