All right I picked up my new Sony head unit today. I already have an aftermarket unit in my car already. So would the old one come out and the new one can just plug right in? The old one if a Kenwood BTW.

I'd go look, but its 2 am here...

Nope. you will have to rewire it/ connect the harness together.

there should be a harness going into the old plug with a plug adapter that went into the back of the old radio, and that will be connected to the harness of the kenwood head unit. you will need to take it out, cut the wires from the kenwood to the plug adapter and then connect the harness of the sony to the pulg adapter harness, make sense?

Depending on how old your Kenwood is, the Sony will just plug right in. They've been sharing the same harness'* for a while now.