Bridging a two-channel amplifier for your car audio system is very easy to do, but if done incorrectly, you'll get no sound from your speaker(s). The term "bridging" means that you're combining two channels of your amp into one. This allows for more power (watts) to be sent to your speaker(s). It's important to keep in mind that when you bridge a two-channel amp, the speaker resistance---measured in ohms---is cut in half (i.e., 4 ohms would go down to 2 ohms). This means that the speaker will allow for more current to run from your amp to your speaker(s) for a louder sound. Some amps are not rated for going lower than 2 ohms, so keep that in mind when bridging an amp.
Research your specific brand of amplifier to be sure that it can be bridged. You will be able to find out whether or not your amp is capable of being bridged down from 4 ohms to 2 ohms.
Locate the four terminals where the speaker wire hooks up to the amp; there will be a positive and negative terminal for each of the two channels of the amp.
Connect the red speaker wire to channel 1's positive terminal.
Connect the black speaker wire to channel 2's negative terminal.
Test your amp and other equipment by turning on your car audio system to make sure that the amp and speaker are working properly.
A two-channel amp will always be bridged into one channel and a four-channel amp will be bridged into two.
Use extreme caution; wiring your amplifier incorrectly can cause severe damage to your equipment.