This post covers 2 topics related to the early cars (98-99) with a 4 channel amplifier.
Later cars had a 2 channel amplifier. They handled balance, fade, and equaliser settings within the amplifier. Communication between the head unit and amplifier occurred over the AVC-LAN bus.
The TSRM wiring diagrams don’t cover the early cars, and most information on the internet refers to the later cars only.
Radio head unit pin out
Thanks to user Adrian Eccles at tezzaworld.com the 18 pin plug on the head unit is already pinned out:
In the past I have successfully used the factory amp and speakers with an aftermarket head unit by soldering RCA positives onto the 4 signal lines. The signal ground is common, so I joined the RCA negatives together.
The amplifier turns on with positive signal voltage like an aftermarket amplifier.
Amplifier pin out
The factory amplifier worked quite well at first. Over time it developed a constant quiet hiss from the speakers, and a speaker pop when turning off the amp. The amplifier did this without a head unit connected.
The harness connecting the amplifier to the car body is standard Toyota. In Toyota vehicles the amplifier is viewed as the head of the system.
The 4 rear speakers in the 8 speaker system are driven separately, with 4 ohm midrange speakers and 2 ohm woofers. Because the mids have a narrow frequency range, and the woofers are low impedance, it is common to replace them with aftermarket 6×9’s.
If you care about impedance:
- Head unit line outs are 10k between channel+ and sgnd
- Amplifier line input is over 2m
- Amplifier speaker output is over 2m on the mids, 3.6k on the woofers
Amplifier turns on at a minimum of +3.3v