I'm not sure if this will help you or not. You'll probably have to play with it a little. Here is an excerpt from the Description & Operation - Amplifier:
When the radio is turned on, the radio sends a voltage signal to the audio amplifier via the amplifier control circuit. The gain control signal circuit allows the radio to recognize that an amplifier is present. Once the radio detects the amplifier, the radio will output low level audio signals to the amplifier. With the amplifier enabled, the signal that drives the speaker is now amplified.
These are pins possibly applicable to what you're looking for:
A6 - PNK - Radio On Signal
A7 - LT GRN - Remote Radio Control Signal (Pontiac)
B3 - DK GRN - Amplifier Control
B6 - PPL - Gain Control Signal
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