I'm looking at installing a small powered subwoofer under my seat. Most conventional guides for doing this suggest putting the subwoofer on either the front or rear speakers to get a mono setup.
My stock radio comes with come front/rear fading based on who you want to hear the audio, driver only, whole car, or backseat only that I make somewhat common use of. For this reason, I'd like to have all four channels going to the sub and mixed evenly. What sort of hardware would I need to mix the front/rear left signal and the front/rear right signal to create two balanced signals that would go into the left/right posts of a powered sub?