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To replace just the head unit leaving the rest of the system in place you will need an adapter to provide the required interface. This is due to the way the factory system was designed.
The 2 most common adapters are:
Metra XSVI-2004
Pac Audio ROEM-VET1
Replacing the head unit should be just plug-and-play by using either one. Both have setup adjustments to customize sound/levels. After tweaking them to your liking, they shouldn't require further touching.