The harness is the whole job
A vehicle-specific harness and fascia adapt the new unit to your car's own wiring. With the right one, your steering wheel controls, reverse camera, parking sensors and chimes carry over and nothing gets cut.
Without it, a fitter starts splicing into your loom. That is where steering controls stop working, where the reverse camera dies, and where the next owner's mechanic finds a mess behind the dash.
Do you even need a new screen?
On a lot of cars, no. If your factory screen is fine and the complaint is sound, the money is better spent on speakers and door damping than on a new head unit.
On some models we can add sound without touching the dash at all. It is worth asking before you assume the whole unit has to come out.
Why door damping matters more than speaker brand
A car door is a thin steel box that vibrates. Fit good speakers into an untreated door and a lot of what you paid for is lost making the panel resonate.
Damping the door first is unglamorous, adds little to the bill, and is usually the single biggest improvement in how the system actually sounds.
Tuning is not optional
A system set up on a bench is set up for a bench. It should be tuned with you in the driver's seat, because that is where you will hear it — and the same speakers can sound completely different once the levels and crossover are set for the cabin.