Hi BRF. It would likely be easier and less expensive to go aftermarket.
I would recommend using Crutchfield to see what is compatible, available and will fit in your Fusion: Crutchfield: Car Stereo, Speakers, Home Theater, Pro Audio, 4k TV
I have used them myself and they are very good. You can use their fitment tool or call.
I like to call and explain what I want, and their customer service reps are very helpful. Once you find what will work within your price range, you can either purchase from them or shop around. If you purchase from them, you get detailed installation instructions, all connectors and harnesses needed for an installation, lifetime technical support etc. Their prices are not rock bottom, bargain basement low, but are very competitive.
On a related note: If you are going to add a sub/amp, you should upgrade and add to your speakers. Doing it with the 4 factory 5x7's would be a little bit of a waste.
Let us know how you make out and good luck.
