This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
Equalizers are normally used to fine-tune a system, and should be
treated as such. Equalizers should not be purchased to boost one band
12dB and to cut another band 12dB and so on - excessive equalization is
indicative of more serious system problems that should not simply be
masked with an EQ. However, if you need to do some minor tweaking, an
EQ can be a valuable tool. Additionally, some EQs have spectrum
analyzers built in, which makes for some extra flash in a system.
There are two main kinds of EQs available today: dash and trunk. Dash
EQs are designed to be installed in the passenger compartment of a car,
near the head unit. They typically have the adjustments for anywhere
from five to eleven (sometimes more) bands on the front panel. Trunk
EQs are designed to be adjusted once and then stashed away. These
types of EQs usually have many bands (sometimes as many as thirty).
Both types sometimes also have crossovers built in.