Stereo, or 2-channel, car audio amplifiers are usually constructed to serve multiple purposes. They can drive two speakers in a stereo configuration, or the output can be bridged to power a single speaker. This is commonly done in car audio to power a single large speaker, such as a subwoofer. No electronic modification of the amplifier is necessary. It's simply a matter of wiring the outputs of the amplifier to the desired speaker. The exact wiring scheme will vary from amplifier to amplifier.
Loosen the two speaker terminal screws on the amplifier that are marked for "bridged output." On a stereo amplifier, there will be two pairs of speaker terminal screws: one pair each for the left and right speakers. When bridging, your speaker wires leading to the subwoofer are connected to one of each of these. The correct terminals will be marked above the speaker terminals.
Slide the bare ends of the speaker wires leading to the subwoofer into the terminals, one wire to each terminal.
Tighten the screws to hold the speaker wires in place with your screwdriver.
Be sure the amplifier is powered off before hooking up the speaker.