tj!2k4: the 03 and 04 harnessess on the mustang are different, hence the different part numbers.. the one i listed has 03 and up but its only 03 and up for other fords/mercurys and 04 and up for mustangs. The back of my radio is a bit different then yours.... I'll try to get a few pics. Thanks for the link on the velcro, I have some of that, I think I'll try that tonight!
ibanez: the 'regular' wiring harness has the wires for the speakers that are non-amplified. just run speaker wire from these to your amp. I believe i hooked up the two negatives together and the two positives together for each channel. If your diagram is right for series, I guess I wired them in parallel. I wired them and checked them with a multimeter for resistence to make sure both were 4ohm so I dont fry the speakers or amp. For the RCA's, you just get an apdaptor that is male on both sides and it hooks up fine.