for bridging it for only one speaker, you can either use the front channels, or the rear channels (2 channels) where you would take the left (+) to the positive to the speaker and the right (-) to the negative end of the speaker. The other two channels are open, I would say get some good full range speakers for the rear deck and run them stereo off the other 2 channels. Just make sure you turn the gain down a bit as to not destroy your speakers.

The only other way to do this would be to have a dual voice coil sub, and bridge front channels to input 1 of your speaker, and bridge rear channels for input 2 of your speaker.