Capacitors
Capacitors are used to store energy for the amplifier to draw on demand. They come in many different sizes ranging from .5 Farads to well over 100 and their intended function is to temporarily cover the short-burst electrical demands of a car audio system that have exceeded the general electrical capabilities of the vehicle.

Rattle-reduction

Sound deadening is often used in the door cavities and boot/trunk area to provide less rattling of the metal in the car, especially the boot/trunk. The most common type of deadening is either Butyl or rubberized asphalt which has an adhesive quality and can be applied by simply pressing it into place with a roller and use of a heat gun (or hair dryer) if needed. Other types of deadening can be sprayed on, but they're less common because of the additional difficulty in their installation. The most popular sound deadening brand would be Dynamat, with others that are not as well known but offer sound deadening products in which some have been proven to be superior in performance such as Second Skin's Damplifier Pro, FatMat, and RAAMat to name a few other companies.