I'm a proud new Avalon Hybrid Limited owner, upgraded from a 05 Prius that's still kicking with 241k miles. The Limited's JBL Synthesis produces very nice highs, but the bass was quite lacking when trying to match those highs so I decided to add an aftermarket sub. After much forum searching and Youtubing, I attempted the install by myself besides the custom box. Since I gained a lot of good info from this forum, I thought I'd give back and share some info. Let me preface this that I've never installed or messed with a car's audio before, so I may have taken a few short cuts vs a professional installer but I think it turned out pretty well IMHO and I saved a ton of money. The audio parts I used were an LC2i line output converter (LOC), JL XD600/1v2, and JL 10w6v3.
: Since I didn't want to mess with the head unit as it probably didn't have factory sub RCA's available since it had the Synthesis, I choose to use the LC2i, a LOC device. I also didn't want to mess with the front dash whatsoever as I knew I'd probably mess it up. I tapped into the signal going to the rear deck's 7x10 which the Synthesis uses as pseudo subs. Unfortunately the 7x10 are inaccessible through the truck so I had to get to them from the backseat. The most "difficult" part was removing the rear cushions, I say difficult as the process kind of strains the back if doing it solo and in 85 degree heat. The bottom cushion pops out easily by simply lifting in the front middle of it, but don't pull it all the way out just yet because you have to next disconnect the heated seats which are on both sides. Next remove the back cushion by undoing 3 bolts located at the bottom of the cushion, disconnect the heated back cushion and lift vertically up on the cushion to clear the hook it rests on. Once removed you gain access to the 7 x 10 speaker wire. The LC2i requires you tap the negative and positives of the left and right speakers as shown in the pictures below. I used the Lockitt Posi-Tap
20-22awg connectors, super easy to install. Note in the pictures I removed the side pillar support cover before removing the seats thinking I could easily lift up the rear deck and access the wires, which was not the case. So I removed the rear seats as mentioned above, so the pillar cover was unnecessary to touch.
At this point, I didn't need to know which wire was hooked up to positive and negative for each speaker as I'll do that in the truck using the 9v battery method, if you don't know it youtube: finding speakers negative and positives using a battery.
Remote Amp Turn-On
: Next I fed the wires through the back seat above where the hybrid battery is located. Most of area behind the seats is exposed once the seat is removed, very easy and wide open access to the truck through this area. I also ran the sub bass remote knob through there too at this time. Last but not least, I ran a wire for the remote amp turn-on through here. I chose to use a tap a fuse
and used the power seats fuse from under the steering wheel for the remote turn on. I hid the bass remote and turn-on wires along the left side floor paneling. Pretty easy as the tucked right underneath the panels. After completed, I reinstalled the rear seats.
Building the Box
: Next I took the vehicle to a buddy who does fiberglass work and had him make the box which took about 3 hours to form and dry, then a few more days for him to get around to bonding the back and carpeting it.
: Once the box was done, I took an evening after work and hooked everything up. I cut out a 3/4" MDF board the width of the exposed sheet metal above the hybrid battery, the height I measured so it'd hang about 1" further down than the sheet metal so I could run the amp's wires through it for a seamless look (see pictures below). I next carpeted the MDF board with some Menard's carpet, cost about $2. After carpeting it I forgot to predrill the holes through the board for the amp and LOC's wires so I did it at this stage, save yourself some headache and do it before carpet. I next mounted my amp and LC2i. The amp I choose was a used JL XD600/1v2. After mounting them, I took them to my trunk and fed all the wires through the holes of the carpeted MDF board, at this point making sure I had my neg/pos right using the 9v battery method for the LC2i, hooked up the remote amp signal wire to the amp and also tapped that same wire to activate the LC2i for power. I ran 4 gauge wire from the battery which is located conveniently in the truck to the amp (first disconnecting the battery). Also ran the power for the LC2i from the battery. Since the battery is so close, I choose to run the negatives for the amp to the battery. Most installs call for the negative going to the frame 2-3 feet from the amp, but most batteries are in the front under the hood, since the Avalon's is in the truck and the car's battery is the best known ground, I choose to use it for the amp's ground without issues. With all the wires in, I predrilled holes through the sheet metal in the trunk and mounted the whole thing up high using 3 sheet metal screws.
I next put the sub in, hooked the power wires to the battery in the back and speaker wires to the sub from the amp. I made sure to run all the wires behind the trunk carpet for that "factory" look.
I'm not 100% satisfied with the sub's speaker connection wires, I might try to run them up and over the back of the sub.
Well, that's about it. For those who may ask how I secured the sub box, well I haven't yet. I'm still thinking about it, but so far I think my best bet will be to attach a thin piece of sheet metal under the sub extending out into the trunk, and secure that to the trunk some how so the sub doesn't slide around. I'm thinking this would also allow for quick removal when going camping where every sq ft of space is needed. It doesn't slide much anyway due to the weight, but extra support is needed. I didn't want to use straps as I've done that before and it looks like crap. IDK, I'm open to suggestions on how to secure the sub, I just want it to be quick to remove, and not permanently alter anything. Speaking of permanently altering, since my Avalon has a warranty to 100k miles, I wanted to make sure I didn't permanently alter anything if needing warranty work. As far as anything permanent is concerned, I only have 3 small screw holes in the sheet metal in the trunk, everything else can be removed within a matter of minutes if needed.
Hope this helps. All said and done, the whole install beside making the box took about 6 beers, aka 3 hours.