Disclaimer/warning: Always have negative terminal of battery disconnected at all times when working on electrical.
Special thanks to @removedonut
for his inspiration and replies to my questions about his install (his is JBL).
Due to the limited space in the truck and lack of real spare tire, I added a spare tire amplified subwoofer. I ended up going with a Rockville Rockghost 8” amplifier subwoofer. I chose this one because it is lower cost, had all the wiring included, and it has a ported design. The SNR is >90db too, which is higher than the JBL, which means more volume per watt. Rockville has a 90 day return policy and free shipping. It shipped fast too! It doesn’t hit low like a sealed 1000 watt 12”, but it definitely is worth the $185 (includes wiring). I have a few tips and suggestions for anyone installing one.
Buy Rockville RockGhost Powered Hidden Car Subwoofer Spare Tire Sub - No Lost SpaceDirect with a 90 day money back guarantee including return shipping
The base doesn’t have an amp and in order to get input, you have to tap the front speakers. While you can do this from the kick panels, it is cleaner and easier to use a harness at the head unit. You don’t cut any factory wiring and you get a remote turn on from the head unit. Thia is what I got. I modified the one end that goes into the head unit. The seller has instructions. You basically shave/sand off 1/8” hump near rear of connector. I neglected to take a photo as I did it last minute. I have arrows showing area. Maybe I could have pushed harder but it worked for me after removing some of the plastic humps. I set the pots on the converter to max. Originally I had them at the default of middle, but the input gain on the amp had to be set all the way up and still needed more. After changing the converter pots to max, all the way clockwise, I have the subwoofer gain at a little past halfway (1-2 o'clock). Having more input voltage going to the sub is probably better anyways as it can mute out noise.
Find many great new & used options and get the best deals for Add An Amp Amplifier Adapter Interface FRS BRZ WRX Impreza 4-Runner Tacoma FJ MR at the best online prices at eBay! Free shipping for many products!
I used this video to remove the head unit. If you find the cables behind the head unit have no slack, they may be wedged under the metal round bar/frame. If that is the case, use a stickor something to free them.
I ran the power cable from the battery using the top terminal of the positive terminal (as this is the one OEMAudio+ showed in their video). I ran the power wire through the firewall using a factory hole that was plugged. This hole could easily fit an 8 gauge or possibly a 6 gauge, but mine was only 10 gauge. The firewall grommet they gave me fit right in. I added some electrical tape to thicken the cable to keep it snug in the grommet. You have to poke a hole through the sound deadening padding on inside of firewall too. I also made a cut on the positive cover to allow the cable to have room with cover on.
I ran the power cable down the passenger side in the actual plastic channel parallel to factory wires.
I ran remote turn on and low level RCA cables down driver side in the plastic channel parallel to factory wires.
I used wire wrap/conduit from Harbor Freight (1/4“ for power and 3/8” for RCA, remote turn on, and remote control cable) for all wires other than what was run in the plastic channels on side. To get to the trunk I followed the factory wire bundles on both sides all the way to the sub/spare tire area. This makes it super easy to route into the thin back trunk panel and between trunk foam inserts and into the spare tire area, bypassing interference with rear seats.