This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
Creating a mono signal is often necessary when you are powering a
subwoofer by bridging the amplifier. Many people do not realize that
bridging an amplifier does not always provide a mono signal - many
amplifiers will simply use only one input channel, which means that the
subwoofer won't be receiving the full signal.
Some amplifiers have a switch that will allow you to combine the left
and right channels into a mono signal. Some signal processors and head
units provide a subwoofer-out channel that can be switched between
stereo and mono.
If you don't have this feature on any of your equipment, you will need
to provide a mono signal to the amplifier. The common thought is to use
a Y-adapter to "combine" the left and right channels. However, by
using a Y-adapter, you are actually summing the line voltages and
directly shorting the left and right channels at the head unit, which
could cause problems.
The correct way to create a mono signal is to cut off the ends of the
RCA cables, combine the signal grounds (the outer shield), and then use
a 1 kOhm (1/4 watt, 5% tolerance) resistor to each of the center
conductors. Solder and insulate the resistors so that you don't short
them prematurely, and then connect the two resistors together. Connect
the summed signal ground to the shield of the new RCA plug, and the
summed center conductor to the center pin of the RCA plug.