I am leasing my 2017 MKZ, and am less than impressed with the factory subwoofer installation (2 6x9 drivers in the rear package shelf driven by a trunk-mounted amplifier; I do not have the Revel audio package). In fact, in my exploration of what it would take to do minimalist wiring of an aftermarket sub, I disconnected the amplifier connector and had a really hard time noticing any effect at all.
As it turns out, most of what is needed by an aftermarket amplifier is provided on the 10-pin connector feeding the factory subwoofer amp behind passenger's side trim panel in the trunk. I'm having trouble uploading a diagram, so I will describe it verbally.
Looking at the wiring harness end of the connector, such that the two pairs of contacts are on top, the top right pin is Subwoofer Line +, and the one to the left of it is Subwoofer Line -. These pins have a 3V offset from chassis ground (looks to be a differential signal with a significant DC offset), so they do not work well connected directly to my Infinity amp's RCA jacks (when turning the gain knob up, the woofer would jump around nastily). The signal needs ground isolation to be used with an aftermarket amp. As I discovered after hooking up an isolation transformer, the signal is too weak. The AudioControl Overdrive Plus provides both ground isolation and amplification in a single awesome package. The bottom right pin is the remote amp turn on lead. It only puts out 6V, but this was enough to trigger both the Infinity and the Overdrive. Next, all I needed was a connector to mate neatly with the factory wiring harness. I searched high and low, but was not able to find where to source the connector. However, a Metra 70-5513 can be butchered nicely to pull power, ground, remote, and subwoofer line leads to the Overdrive Plus. Disassembly of the connector and moving some of the pins around is required, but it's very easy.
The "arch" trim panel that spans the trunk conceals a nice steel support with a circular punch-out that should work to mount the sub with no drilling. Now all that's left is to install larger power and ground wires. Woohoo!