The 99 harness will not have the grounds for the O2 heater circuits or some other grounds required by your 04 PCM. As Pocono said, the 99 has red retainers instead of the green ones your 04 PCM needs, they wont fit either since they are keyed differently. Regardless, if the PCM and harness match (whether 99 or 04), they will work with the 04 engine albiet if you change the FI connectors.
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Daily Driver: 1986 El Camino LT1 w/4L60E, 3.73's, 3850-lbs, stock, 13.8x@100.xx on Street Tires (2.2-60 ft)
Fun Driver: 1964 Biscayne LT1 w/4L60E, 2.73's, 3800-lbs, stock, 14.2x@96.xx |