In the second part of my sub install in a Mercedes CLS or E class I mounted the amplifier. I wish I had never done it this way but take this post as learning from some of my mistakes and use the benefit of my retrospect for your install. If you want to skip this part and read about cable management check out part 3.
I mounted the amp to the ceiling of the trunk, before the trunk lid, on a black mount made out of some ridiculously hard material. Something so hard you can't really thread into it. So if you are going to mount on this, which in retrospect I wouldn't, it is best to drill a hole and have either a metal fastener at the end, or maybe a slick bolt setup?
There is some room further "in" (fore) the trunk. If you really want to mount on the ceiling I would do something here carefully with like 1/2" sheetmetal screws and perhaps some spaces depending on depth. My amp was too big to mount here.
What I should have done in retrospect was mount the amp to a side fairing to the side of my subwoofer. I would likely attach an amp fairing piece with the up to 2x10" subwoofers (2x10") with some sort of dampening between, like padded rubber washers, and another fairing for the other side of the subwoofer. Both can be attached to load space loops near the back seats with strap ties.
Overall I wouldn't mount to the black thing again, or the ceiling as mentioned. However I did and it was painful. I used a 3/4" piece of MDF mounted to 4 screws with metal fasteners and 1/4" rubberized plastic spacers. I mounted the amp to this and in retrospect i would have set the threaded inserts. I thought the dude on youtube suggesting it was adding complexity but it would have been useful as holding the amp and drilling holes and then hand screwing the screws in so you don't strip them.
The wiring I still have to work out. It started to pour so I didn't really even get a chance to clean up, not just ruining my prior car wash.