How simple is a head unit?

[Slam Wagon]

Im no good with electronics, but ive had an idea. How simple could you make a head unit?

I dont use the radio an barely ever use my CDs is only really the aux input on my headunit so i was wondering if a custom one could be made.

Ideally with a usb input so it would charge at the same time but an aux input would be nearly as good. An then just a volume control, maybe an on/off switch.

Is this posible? How small could it be? How tucked away could i get it? Could i just have a single dial on show somewhere for it?

I know theres not really any gain to doing this but its one of those random ideas i get and i think it could be cool.

Im thinking of replacing my top glove box with a custom ipad holder so maybe i could just put the controls in the edge of that.

Anyone on here with an answer to any of my questions would be sparking some creativity.

Thanks

Ian