Do you want a harness to add a new HU to your system or do you want to add am amp to your factory HU?
I have a harness that will let you add an amp (replace the factory amp with an aftermarket) to the Premium and NAV HUs. My harness provides the factory HU preamp outs to an aftermarket amp. The factory HU pre-outs are balanced differential and only work with amps with balanced inputs. Crutchfield has a harness to tap the speaker wires so you can replace the HU and wire your speakers directly. If you want to upgrade your amp you can use both harnesses and you won’t have to cut or tap any wires.
Since you have an Si you already have the premium sound HU. This is a good HU when coupled to a good amp and speakers; it is not your run of the mil cheapo factory job.