Not to make a dumb comment, but are you sure you have an amp?
If you look at the back of your main fusion unit, you either have speaker wire connections coming out of it, or RCA wire connections coming out of it... If there are RCA cables you can track them to the amp, if it's the factory speaker wires, they're going into a harness.
I found plenty of room behind the microwave when I mounted an amp. But under sink, behind fridge, or in the port aft cabin side would also work and be an easy run for RCA cable outputs from your head unit.
It is unlikely (but not impossible) that someone would have gone to the trouble of using the speaker-outs as inputs for an amp and hacking up the harness... my guess is your fusion head unit is powering the factory speakers directly though the factory wiring harness that terminates in the top cabinet above the fridge.