When you say "automatic volume" are you meaning the Speed Compensated
Volume feature? If so, there is a signal line in the radio wire
harness (labeled VSS in my 2001 Camaro service manual). This is the
signal which tells the radio how fast you're moving. Which pin it's
on might vary according to harness and plug configuration. I have the
21 pin plug which is connectors C1 & C3 combined (C2 is not used in my
vehicle). I'm not certain how this signal is composed. It might be
variable voltage, or it might presetn a varying resistence, or more
likely it might be a serial data signal (similar to those used for
steering wheel radio controls or remote CD changer control).
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