I’ve never been one who’s able to leave “good enough” alone, especially when it comes to electronics. I like deep bass in my music. I outgrew my desire for twin 12-inch subwoofers decades ago, but I still like a deeper sound than what most factory stereo upgrades can give. I’ve been satisfied by various single 10-inch subwoofer systems for over 20 years. So it was a natural pursuit when I decided I wanted to upgrade my GTI. Of course, I “had” to add a ham radio and make the entire installation more complicated than necessary. Read on to see what I did…
I had been fairly pleased with Volkswagen’s MIB II base stereo. It’s no powerhouse, but it’s still adequate. I had originally planned to simply tap into the rear speaker wires to feed a signal converter, an amplifier, and then a subwoofer. I really think I would have been satisfied with that relatively simple setup. Then VW decided to discontinue its Helix subwoofer accessory, meaning they slashed the pricing by more than 50% to quietly clear their inventory. What was once an “is it really worth $660?” item was suddenly a very affordable $300. I managed to get the kit shipped to my door for $235 after some discount shopping and a manufacturer’s rebate. The wiring harness and digital signal processor alone are worth more than that! The Helix subwoofer features two 5-1/2″ dual-voicecoil subwoofers and requires permanent removal of the spare tire. So I really had no intention to keep that setup in place after the initial installation and test.
Instead, I routed two pairs of the subwoofer output from the Helix amplifier into an Audio Control EQL graphic equalizer with subsonic filter. The EQL accepts the speaker-level input, shapes the signal according to my settings, and then outputs line-level signals for my Rockford Fosgate T500X1BR Power 500-watt monoblock amplifier. I chose this amp because of its small size and forced ventilation. I’m using a Rockford Fosgate Punch P3SD4-10 10-inch shallow-mount subwoofer since one of my goals is to keep my spare tire and still have room for my other electronics. The subwoofer limits me to 300W RMS (600W peak) and works in an enclosure as small as 0.4 cubic feet. I may eventually upgrade to a T1S1-10, but so far I’m completely satisfied with the P3SD4-10. I use a PLC2 Remote Punch Level Control so that I can fine-tune the subwoofer levels from the driver’s seat. It’s mounted just inches from the parking brake handle. I’ve shared a photo at the bottom of this page.
The box itself is rather unique, mostly because it’s more than just a box. I wanted everything to be concealed beneath the removable floor in the trunk. “Everything” includes the sub, power distribution, amplifiers, ham radio, and any peripherals that I may add later. On top of that, I wanted it all to be removable in case I ever decide to autocross or track the car. So I made a full panel with integrated sub box and quick-connections for the electrical parts. The 3-3/4″ tall box has ~0.52 cubic feet remaining with the subwoofer mounted. I used Acousta-Stuf polyfill to fake more volume. The 68-lb panel can be lifted to reach the spare tire and whatever is left of the storage area; or it can be removed altogether. Getting the carpet applied evenly was my greatest challenge. It’s not perfect, but I think it turned out well.
I ran 12V directly from the battery via 4-gauge wire to a 120-amp relay on the driver’s side of the panel. The relay is triggered by a voltage controlled switch, the APO3. This device allows battery power to feed the rest of the panel so long as its voltage is above 13V, basically any time the car is running or for about 30 minutes after shutdown. Once the battery voltage falls below a preset threshold (I can set it for 13.05, 12.7, 12.1, or 11.8 volts) for a preset time (0, 5, 10, or 20 minutes), the 120A relay loses its trigger until the car is restarted and has run for a few seconds. The APO3 assures good battery health and no surprises as a result of a dead battery (been there, done that). From the relay, 12V travels to a fuse block with circuits for the amplifier, a “Super Booster,” an ID-5100D VHF/UHF digital amateur radio transceiver, a Raspberry Pi3 w/DV-Mega (not pictured), a 400W pure sine wave voltage inverter, and a second APO3 (which is set to power the EQL and switch the amp only when the car is running). It’s overkill for my application, but it should never act as a weak link in the system.
One item that’s often overlooked in such an installation is that of safety. I didn’t think about the potential for flying debris when this stuff was mounted in the trunk of a sedan. I’m compelled to consider missile hazards now that it’s mounted in a hatchback. The panel seems relatively secure, given that it will only open about 15 degrees unless I pull the panel rearward (which requires the hatch to be open). Still, a crash will likely break any plastic bits that appear to be retaining my panel beneath the false floor. I chose to use an OEM rear cargo net, P/N 5N0065111, to offer some peace of mind. I suspect that its hooks could break in a nasty accident, but I think it would take a lot. The idea is to either restrain the floor’s movement or at least slow it down so nothing gets airborne. To summarize, the 68-lb electronics panel would have to lift the cargo floor, overstretch the net, break past the factory plastic floor supports, snap the net’s hooks, tear out the 4-gauge battery/ground wires, rip out the coaxial cable (ham radio transmission line), and blow past the cargo privacy lid before I would need to worry about it hitting me or my passengers. That’s a lot! I’ll have worse things to worry about if I get into that bad of an accident.
In short, I replaced the Helix “subwoofer” with a system that features a speaker with three times more surface area, handles more power, plays deeper bass, and retains the spare tire. Bonus: A 10-inch subwoofer in a hatchback provides even better bass than what I had previously enjoyed in sedans since there’s no “trunk” acting as a barrier. I have shared more photos of my project below and I have addressed the amateur radio installation on a separate page. Feel free to ask any questions about my rather unusual setup.