The subwoofer mount is a little less straightforward. It is secured to a small captive bracket under the dash, and the other side pushes into flexible stopper that wedges into the front of the passenger footwell:
View attachment 1389395

The relay is mounted on a bracket that uses two holes near the center of the dash, near the ODB port:

Note that I still have the OEM cupholder that is functionally useless, but is whimsical and cute.

I concealed the power wire from the battery and the backup camera cable in the main harness:

Initially, I planned to run new speaker wires, but decided that it added too much thickness to the harness that runs through the tunnel. So I ran line level inputs to the amp, and high level outputs back to the factory harness. Yes, the OEM speaker wires are crappy, but this installation is full of compromises. The harness goes through the slot at the edge of the trinket tray on the drivers size. Getting the wires in requires that the left side vent and starter button trim and side sill get removed.

Similarly, the subwoofer line lever outputs and backup camera cable run on the passenger side, together with the head unit's USB cable.

This is what the under dash wiring looks like on the driver side:

And the passenger side:

I secured the power wire from the relay via a little zip tie and a small 3D printed bracket that uses an existing hole to make sure that it clears the steering column universal joint.

The installation was way more work than I anticipated and required a ton of disassembly.

The project was fun, but it is not for the faint-hearted! Overall, it is a significant improvement in the audio, although still not exactly high end. I would say that the sub is the weakest element, but then the installation location is far from optimal.

 

I forgot to mention.

The power, radar detector, rearview camera and power socket wires were run on the left size of the shifter and then under the main harness in the forward tunnel.

The ground that goes to the relay is attached to a bolt that holds down the front of the shifter mechanism. It is partially hidden in this picture:

Also, the ignition-on signal for the relay is taken from the C3 cabin fuse, using a Tap-a-Fuse:

This is the power cable and rearview camera running into the grommet at the bulkhead:

I used the same zip tie locations as the factory harness behind the seats:

 

Excellent! Lots of great information! Thank you for sharing.
I am at the decision point going the same way you did or a more minimalist approach tossing the head unit and using a bluetooth receiver with a volume knob to output the sound directly to a small amp and small sub, both with some sound adjustment capacity. I will reread your thread! Did you upload your 3D files onto Thingiverse or equivalent? Would be very helpful.

 

I was also originally thinking of tossing the HU, but the backup camera display was the big draw for me. Navigation too, since I use the phone to mirror the V1 radar detector.

I have uploaded the speaker ring files and the other component mount files to Thingiverse. The descriptions might be a little brief, but I am willing to add more if needed.

 

I am revisiting the great info in this post. I went a different approach since my goal was to get rid of the Radio head unit and save weight while having better sound. I have the rearview camera on the digital dash, so I didnt need the head unit for the rearview camera.

I installed a Kenwood KAC-M5024BT in the space in place of the head unit. This amp has a bluetooth module with volume and equalizer and subwoofer gain control. It is also marine grade so vibration/ waterproof.

Using a stealth approach too, meaning all the dash panel came off. Power and ignition power are all tapped from the wiring underneath the dash. I also decided to use the OEM speaker wires for simplicity and for the low benefit they would provide.

Subwoofer wires go through the large dash grommet and it will rest in the passengers feet area.
I have a subwoofer but would love to install something more compact that I can unplug and take out at the track.

@ Paul M.: Could you please upload a few more pictures of how your subwoofer fits in the footwell? I like its small size but would like to understand a bit better how it fits there and how your 3D printed brackets work.

 

@rleg, that seems like an awesome solution. I'm sure that a backup camera would be a lot better viewed in the dash than the head unit. One question: have you considered the power consumption of your amp? It seems to need 26A so requires a 40A fuse and a 10AWG power wire. I doubt that there is this amount of current in any source within the dash. This is why I ran a wire from the battery and used the Eastern Beaver for current distribution.

Also, how are you going to secure the amp? Double sided adhesive tape?

used a two main mounting points for the sub: one deep on the footwell and one on the dash trinket tray.

This is what the footwell mount looks like:

The blue part is a grommet made from TPU, a flexible rubber like material that pushed into the wedge-shaped slot in the front of the footwell. The orange part is fastened to one of the subwoofer mounting screws, and is attached with a hinge to the rest of it.

This is what the trinket tray mount looks like:

The blue part is fastened to the other subwoofer mounting screw. The gray part is fastened to the tribket tray and provides captive nut that allows the blue part to be fastened to it.

Here is the subwoofer with the mounts attached. The thing marked in red is a TPU pad that goes between the edge of the subwoofer and the aluminum extrusion of the chassis tub.

I also used some foam padding where the side of the sub presses against the car:

To mount the sub, the footwell mount and grommet are pushed into the wedge shaped slot next to the big wiring harness, then the sub is swung upwards on the hinge:

The trinket tray side is then fastened to the portion already fastened to the trinket tray. You can just see it here:

Here is the best view of it all together:

It is held tight against the tub by the mounts.

Again, not sure that this solution is for everyone. I think most folks would choose to replace the footrest with a sub in its place.

 

Great, thank you for the pictures!
-Regarding power supply: There is actually a power post on the driver side fed by gigantic battery cables: It is only 1 foot away from the amp so I didnt need to add nay wiring to it: Just crimped a large eyelet connector, and screwed it to the post. I used the same source for the amp power supply.

-For the amp, there were holes I used to pass zip ties, plus some 3M double face tape all around. No vibration and wont go anywhere even in a crash!

I am impressed by the sub mounts and the use of the the flexible printed TPU!
THE question now; How does the sub sounds? I did read mixed reviews, but people rarely have such a small cabin space1

 

Wow! I thought I had done a lot of research on audio installation, but had never seen any reference that power post.

Regarding the subwoofer, I would say that is an incremental gain if anything and I doubt that it makes a huge difference in most situations, especially if you have beefed up your speakers. Of course there are settings to play with such as gain and crossover freq, and I have not spent much time adjusting them. If you can find a bigger subwoofer that fits your aesthetic requirements, I would encourage you to consider it.