You need to kind of wiggle your harness to the left, right, or below the head unit itself. It can't be directly behind it or it won't fit. I've changed out my headunit a good number of times between two cars (most recently about a week ago in a gas station parking lot
). I usually find a hole on the left side, push the harness over into it, and slide the HU straight back.
It's just a matter of messing with it. You'll get it.