This is getting complicated now.
Forget what I said about amp bypass. More research material below. Is your amp getting the turn on signal from the head unit? This desciption says it may need 12v taking from the aerial turn on connection of the new head unit.
Power amplifier.
Some cars have a power amplifier, these amplifiers are switched on by a +12V turn on signal, this is the same signal as the antenna turn on. The amplifier receives a normal audio signal from the basic head unit or tuner radio module and then drives the speakers in the car. Cars with the power amplifier will have single speakers in the rear doors, three way component speakers (low, mid and tweeters) in the front doors and a centre speaker on the dashboard. The centre speaker is bridged between the front door mid range speakers.
View attachment 244259
View attachment 244260