99.9% sure you'll need the wiring harness. Someone can correct me if I'm wrong.

 

You'll definitely need a wiring harness unless you like cutting wires in your car just to save $10. :screwy: I'm not sure about mounting in your '04, but my '09 was less than a perfect fit for the 810 I bought. Sure, it bolted in perfectly with the mounting hardware provided but needed to do some cutting to get the face to detach and flip to insert discs.

 

Yes... you need a harness, or more specifically an "adapter" to convert your factory wiring plug to the JVC stereo input. They are usually $10-15.

What you DON'T need..... is an "installation kit"..... and they will try to sell you one. The factory WRX stereo mounts are extremely universal, and you can just bolt-in and single or double-din stereo.

Be aware of anything that folds or slides out. I had an issue with a fold out pioneer a couple years ago. It used to bind on the stereo panel when it folded out.

I relocated it from the lower din to the upper din.... problem solved.