This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
If a proper center image isn't achievable via a two channel
configuration, installation of a center channel can help. Since the
majority of recordings are done in two channel, a two channel system
designed correctly should be able to reproduce a center image which was
captured during recording. A center channel is not simply a summation
of the left and right channels, like bridging an amplifier; rather, it
is an extraction of common signals from the left and right channels.
This usually means the lead vocals, and perhaps one or two instruments.
These signals will then be localized to the center of the stage,
instead of perhaps drifting between the left center and right center of
the stage. A signal processor is usually required in order to properly
create a center channel image. The image should then be sent to a
driver in the physical center of the front of the car, at an
amplification level somewhat lower than the rest of the speakers. The
correct frequency range and power levels will depend on the particular
installation, though a good starting point is perhaps a pass band of
250-3000Hz at an amplification level of half the power of the main
speakers (3dB down).