My car is an auto but for a 2007 manual you can press the stop/start button twice making sure you don't depress the brake or clutch pedals and the ignition is switched on. You can then listen without the engine running
There are devices you can buy that sense a signal on the speaker cable and that puts feed through a wire that can be used as a remote turn on. If you have RCA cables from the back of the head unit to the amp this won't work. I tried such a device and found it switched the amp on and off when doors where opened or closed and that make a loud bang on the speakers so I disconnected it, probably my fault for buying the cheapest

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