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I came up empty-handed while searching for this information on the internet. So I decided to figure it out and then post it here, and on a couple of other forums, for others that might find it useful. By probing and measuring my 2019 Tacoma TRD Sport w/JBL sound system with some test equipment and a lot of patience, I managed to discover most of the JBL amplifier connectors pinouts.

On the bottom white 24-pin connector, the white & black, and green & red come from a shielded bundle and are the left and right audio channel inputs to the amp. They are differential audio signals and not single-ended. Therefore, each channel requires 2 signal lines instead of using a common ground. Those are the only audio inputs to the amp as far as I can tell. The amp not only amplifies but also generates the individual front, rear, and front dash (mids/tweets) speaker outputs, and they are all derived from just the right and left signal inputs to the amplifier. You must keep the factory JBL amp in the circuit in order for the factory head unit to function properly. Also, the head unit only sends a constant audio level to the amplifier which does not vary according to the volume that you set on the head unit, which is counter intuitive. The volume and fader levels actually happen inside the amplifier, not the head unit! The head unit has some command/control over a data buss that it uses to communicate to the amplifier for things like volume, fader, muting, etc. What does it all mean? Well, you can start thinking of the factory JBL amplifier as nothing more than an extension of the head unit itself. If you plan to use the factory head unit with aftermarket amplifier(s), you'll need to keep the JBL amplifier in circuit, take the speaker level outputs from the JBL amp, and run them through line-level converters before finally running them to your aftermarket amp(s), unless they already have speaker or high-level inputs. Here are the power, ground, and speaker outputs of the JBL amplifier:

top white connector on OEM amplifier:
large white/black wires (2) are the system ground (vehicle chassis ground)
purple/green = subwoofer voice coil1 + (pos)
purple/yellow = subwoofer voice coil1 - (neg)
purple/black = subwoofer voice coil2 + (pos)
purple/red = subwoofer voice coil2 - (neg)
green/yellow = front left door speaker - (neg)
green = front left door speaker + (pos)
blue = front right door speaker - (neg)
gray = front right door speaker + (pos)


middle black connector on OEM amplifier:
large white and large gray wires (2) are the constant +12 Volts supply to the amplifier
violet/blue = right rear door speaker - (neg)
purple/green = right rear door speaker + (pos)
white = left rear door speaker + (pos)
black = left rear door speaker - (neg)
red = left front dash speaker (mid/tweet) + (pos)
yellow = left front dash speaker (mid/tweet) - (neg)
pink = right front dash speaker (mid/tweet) - (neg)
violet = right front dash speaker (mid/tweet) + (pos)

This should be enough information to enable you to easily add an aftermarket amplifier and subwoofer to your 2019 (and possibly the 2020) Tacoma TRD with JBL audio system. Hope this information saves someone else some time...
 

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Thanks for this work! Any links here on tacomaworld with dsp intigration and/or relocation of the factory JBL amp. Looks like I will need to extend the harness and move my amp from behind the rear seat. This is a project but I need to retain the stock head unit on my 2022 Sport.
 

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Thanks this post was extremely helpful and confirmed a lot of what I was suspecting about jbl oem systems in general. I was coming to a similar conclusion but was worried about not being able to find a full range speaker output to use as inputs to the aftermarket amp, were you able to find something that worked? Im also not super thrilled about layering on whatever processing was done in the oem amp into further processing in the aftermarket amp. Would be especially problematic if you had to combine multiple output channels to create a single full range input as the time alignment and levels would likely be off at the different frequencies.

i have found PAC Audio makes “integration interfaces” that take the fixed level inputs and the CAN data connection from the infotainment units to layer in volume control/fader/etc into analog inputs to use with aftermarket amps, but not yet sure if they work with JBL systems w/ harman becker amps. I have sent them a note asking about the specifics of my setup so we’ll see what they say.
 
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