You can install it without a harness adapter, but that can be a real can of worms. Harness adapters are easy to install and make going back possible if you decide to sell the vehicle and want to keep the new audio system. and they are just about foolproof. They don’t cost very much either.
Your new radio should have the plug that goes into it, the wires are color coded with the universal color code for aftermarket radios. You get one of these and you crimp or solder the wires together by matching the colors. Then this adapter connects to the plugs that used to plug intot he back of the factory radio and you are on your way.
You can also find faceplate adapters that will make the aftermarket radio look like it was installed at the factory. Not all vehicles need this though. Mostly this is for vehicles that had an odd size opening for the radio.
If you are replacing the premium factory audio system, this is the adapter you will need. The premium system has remote amplifiers.