I suggest you steal unregulated DC voltage from just after the bridge rectifier on your engine's electronics. In round numbers you won't be stealing more than, say, 5 Watts of power for any reasonable audio amplifier application. This should not be an undue burden on the bridge rectifier. Then use one of the 99 cent DC-to-DC regulator modules to generate 7.5-12V DC - it should have several hundred uF of capacitance whether already on-board or added to it if it doesn't already have it (e.g., the DC-to-DC regulator board GRJ shows on the left).
If done this way, the new amplifier and the source of the audio will share a common ground. Some folks will claim that's why you put a coupling-capacitor between the source output and amplifier input. That's correct, but not the entire story. Without getting into the nuts and bolts of hum-rejection from floating grounds, I'd just go with a common ground.
