Installing a new headunit, quick question... Options

[Benevolence]

I am going to be installing a new headunit today or tomorrow and I currently already have a aftermarket headunit that was professionally installed 4 years ago.

What I would like to know is since I already have had a aftermarket head unit installed will it be a plug n' play operation with the existing wiring harness or not?

[absolutefunk]

Only if the new HU uses the same harness as the old one. A wiring harness for an aftermarket HU consists of the half that comes with the HU (that plugs into it), and a vehicle-specific connector that connects to the stock connector (this part you have to buy if you have a stock HU, but you already have this as you have an aftermarket HU installed already). These two halves are wired together to form a harness from the stock connector to your HU's connector. If the harness (the end that plugs into your new HU) that comes with your new HU is different from the old one, you'll need to first remove the the old one from your vehicle-specific connector and then re-wire it to your new one to make the new harness. This process is real easy, just kinda repetitive as there are many wires to splice. If you are using an external amp(s), then you can skip the internal amp wires, which cuts down the splicing by like half lol.