False Floor Sub Install
Hey ya'll wanted to take a minute to show off the project I worked on last weekend. The lack of bass is a common complaint with these vehicles and coming from my previous vehicle that I added an extra 12" sub it is especially noticeable. As an added challenge, I was tired of losing half my trunk/cargo area to the sub, so having something mostly integrated and out of the way was a must.
Things you'll need if you want to attempt:
A T-harness and optionally a Line-out-converter (if using an amp that doesn't accept high side audio signals). I sourced my harness from t-harness.com. 2024 and 2025 MY non-Bose systems will use this black and gray harness, but always visually verify the colors needed before hand.
1x 4'x4' 3/4" MDF and 1x 2'x4' 1/4" sheet of MDF (Cut sheet below).
1x Sub, the total stack height needs to be < 4.5", and mounting depth < 3.5". I went with a Pioneer TS-DS10LS4.
1x amplifier, I used this cheap Boss unit.
1x speaker terminal cup, I reused the one from my old sub box.
Wire: ~25' of 4ga for connecting the amp to the positive battery terminal and the amp to ground
~15-20' RCA cable, if using a LOC and low side audio
~100' 16-20ga wire for high side audio connections (if not using LOC), if using LOC you'll still need ~15-20' to connect the ignition sense wire from the LOC to the amp.
Prepping the vehicle:
Starting on the front passenger side you'll remove the trim piece that extends from the bottom of the door towards the foot wells. This lets you peel back the passenger footwell carpet so you can access the four 7mm hex headed trim screws that hold on the passenger side kicker plate. Two of the bolts are towards the front, while the other two of them are hidden far back in the foot well, you will need ~6" extensions for the rear two screws. Remove the front two bolts first, then disconnect the wire harness connected to the footwell light and free the harness from the plate before proceeding to the rear screws.
Once the kicker plate is removed, looking up, out towards the sideview mirror, and towards the front of the vehicle you'll be able to see the audio connectors, they'll either be black and grey or aqua and tan. These are the connections you're going to intercept with the T-harness. Remove each connector (note which goes where) and add the T harness.
Sources note that some Blazer EV radio modules have ANC and will require a bypass when adding a sub, I haven't noticed any issues in the past week but it is something to be aware of t-harness.com also sells the potentially needed ANC bypass. If using a LOC, attach the RCA cables to it, and zip tie it up and out of the way so we can put the carpet, trim, and kicker plate back without it interfering. The RCA cable and ignition sense wire can be run along the door trims until you get to the flaps that cover the gap between the cargo area and the rear seats.
Moving over to the driver side footwell we'll repeat the kicker plate removal process, which is thankfully much easier on this side. The hood release latch will prevent you from fully removing the trim along the door, but it doesn't need to be fully removed like the passenger side. Removing the kicker plate on this side reveals the steering shaft boot, which is what I used to pass the 4ga power wire through the firewall. To do so I made a small ~1/2" incision in the rubber away from the shaft and pushed a length of the 4ga wire through the gromet with a pair of long reach needle nose plyers.
Next we'll head out to the hood, remove the decorative plates (if equipped) and the frame that holds them. I had to remove the battery and brake fluid reservoir to get my long reach pliers back far enough to reach the wire I poked through, this may vary between '24 and '25 MY. Attach an appropriately sized fuse and holder (100A for a 1100W amp) on the 4ga cable, but do not connect it to the battery yet.
The frame holding on the decorative plates on early models
Rear of the firewall, 4ga wire coming through top of the steering boot
12V battery removed and brake fluid res dismounted but not disconnected to access firewall
Back inside, run the 4ga wire along the interior under the door trim, until you reach the cargo/backseat gap. The dead space between the rear seats and the cargo area is where the amp will live, secure it to the existing rear seat bracketry to prevent movement.
Amp located in the dead space behind the rear seats
Once in place, cut your wires to length, leaving some slack for future service. Using a T55 bit, remove the bolt securing the rear seatbelt to the floor (either side works but I used the passenger side to reduce the chance of the ground wire rubbing against the positive wire and shorting), securely attach a section of the 4ga wire to the bolt with an eyelet and then retighten to ~40ft/lbs. This will be the amp's ground connection.
Passenger side rear seatbelt bolt for amp ground
Heading back to the trunk. Remove the dead space cover by pulling up firmly to disconnect the metal clips. Remove the false floor as well. The plastic trim can be removed after the four 10mm nuts. With the trim removed, cut out the left and right bottom of the back section of the trim to open the space to 12" x 36". This can be done carefully with a razor blade, but an OMT will cut the time down significantly. Now is also a good time to drill a small hole in the trim from the dead space into the false floor area to pass the wire from the amp to the speaker cup. Before reinstalling the trim, cut the carpet and 1" thick sound insulation out from the bottom of the cargo area. This gives us the extra depth needed. Add 5mm adhesive sound insulation pads to the now bare metal underneath. Do not relocate the sensor in the top image below, it is the rear keyless entry sensor and moving it just a couple of inches will cause a "Service Keyless Start System" error, though the car will start and drive normally despite it, just an annoyance.
False floor support trim reinstalled, with the front two mounting points cut out.
Build the Box:
I'm not going to go into depth here but the cut sheet is as follows:
Top (sub mount point): 3/4" x 12" x 36"
Bottom: 1/4" x 12" x 36" I added additional 3/4" x 3/4" x 12" bracing along the bottom, but being secured to the floor of the vehicle should provide more than enough support for the thinner bottom.
Front and Back: 3/4" x 34.5" x 3"
Sides: 3/4" x 12" x 3"
Cut out the hole for the sub, 9" in my case, and an additional hole for the speaker cup but do not install the sub yet. Attach the wires that will go from the speaker cup to the sub before you attach the top panel to the box.
Mounting the Box:
Reinstall the false floor trim using the 2x mounting points still attached. Add a small dollop of paint, red and tacky grease, etc to the tops of the 8mm studs and carefully lower the box into place. Lift the box straight out and drill 5/16" holes at the markings. At this point you can carpet the sub box before reinstalling. Attach the wires from the amp to the speaker cup prior to installing the sub box, it is a massive pain after the fact. Use a rubber mallet to tap the box down onto the 8mm studs so that it sits level. Carefully fasten the 10mm nuts back onto the studs using the opening at the top of the box, go slowly and tighten each side equally as the center of the floor is higher than either side, overtightening can break the MDF. At this point you can install the sub.
Now, because we cut out the front two mounting points for the false floor trim and removed 1" of support from underneath it, we'll use two small L-brackets to fasten the top of the false floor support trim piece to the side of the sub box, raising the trim enough so that the false floor sits even and up off the protective rim surrounding the top of the woofer. This prevents the false floor from being rattled by the sub and road bumps.
Testing and Clean up:
Wire the amp based on the manufacture instructions. Attaching the fused positive 4ga wire from the amp to the battery should be the last step, it will spark but its much easier than trying to secure the amp ground to the seatbelt bolt last. I connected it to the readily accessible 10mm nut under the red terminal cover. Verify the sub is working, adjust gain, low pass and any other settings on the amp to your likening. Reattach any loose trim, and the kicker plates (don't forget the light and its harness on the passenger side) and you're off to the races. Make sure you abide by the recommended break-in period, in my case 24-48 hours of normal listening before I can crank it.
