Sorry about the confusion. I'm going to be connecting an amp and a car radio to the same speakers, so that I can use the car computer with the amp and my dad can keep using the crappy old car stereo.
Then what I'd do is get DPDT relays (one per speaker) and wire them up like this.
NO1 -- amp speaker(+)
NC1 -- radio speaker (+)
NO2 -- amp speaker (-)
NC2 -- radio speaker (-)
Com1 -- speaker (+)
Com2 -- speaker (-)
Do this for each of the speakers and be consistent with your wiring as far as if the amp or radio is on the NC or NO terminals or you could have a mess. Once you have all of this done, wire the coils of each of the relays together and run one side to ground, run the other side to one terminal of a switch and the other side of the switch to 12V. Then when you have the switch off the radio will be connected to the speakers, when the switch is on the amp will be connected to the speakers. Enjoy!!