The easiest way is to just use a small amp in place of the stereo. The pc will go directly to the amp and then to your speakers. In a MX5 you only need a 2 channel amp for the fronts, the headrest speakers are a joke and completely destroy any imaging you might have had.
In this case, the cables you would need are mini-jack to RCA to go from PC to amp. Then some speaker wire to go from the amp to the door speakers. You could run the speaker wires from the amp to the factory harness, but it will probably be too much power for the 22awg factory wiring or the factory speakers.