You can use that adapter. The one he linked you to attaches to the harness, not the radio. Those bare speaker wires would be the ones you would use for the speaker level input to your amp. Also, you might want to look at using a line output converter. There usually used to convert speaker level output of a factory radio to RCA signal level. Possibly they can be used backwards? Then you wouldn't need to cut up any wiring. The signal flow would be: PC--> 1/8" stereo to RCA cable--> Line out converter--> Wiring harness--> car wiring--> factory amp. Also, you may want to look in yout manual for some sort of remote turn on lead that the factory radio used to turn the amp on.