Off-topic: question on custom wiring car audio controls

This may not be right forum for this question, but it's the only audio site I'm signed up with!

I'm putting a together a basic stereo for my car and need some help in wiring the controls. I have all of the components, however it needs a volume control, on-off switch, and selectable inputs (3.5mm jack, Bluetooth, and possibly USB). Any thoughts on where--or who--I can go to for help on the wiring?

I'm auditioning speakers from Focal, Infinity Kappa, and JL Audio. Anyone have any thoughts or experience with these?