This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
The amplifiers are all connected in one way or another to the battery.
In fact, unless you're running separate power wires to each amplifier
all the way from the battery, they're usually connected at a more
proximal site (a distribution block, for example). The effects of the
capacitor are felt by the entire electrical system, including the
amplifiers. Therefore, you cannot selectively dedicate a capacitor to
a specific amplifier.