Quite agree with the facia plate trim pieces, but haven't spotted any yet for mine.
this is my current interior:

 

Only one wire from my '99s harness for steering controls connected to one wire on HU. Couldn't be easier. Program what you want each button to do in about 30 seconds from the HUs settings menu. I programmed them to operate the same way as the factory setup.

Mine is a generic Chinese head unit. Not sure about the typical big name aftermarket units. Those may or may not require an adapter. Some of the later models might have a more complicated network communications scheme as opposed to the varrying resistors in my steering controls circuit. Look at your specific wiring diagram and how that circuit works should be fairly obvious.