For harness adapter info -
see this thread , particularly post # 9.
Here is the part you need - http://www.millionbuy.com/a-ihwh804.html
The one illustrated in post # 9 of above reference is for after-market head & stock boase amp/speakers.
For an external amp and aftermarket speakers in addition to the new head, you woudn't need the RCA pigtails & instead could use the OEM wiring that originally carried the pre-amp signals to the door speaker/amplifier assemblies for your speaker drives; you can integrate your speaker wires onto that adapter as opposed to running new speaker wire all the way to inside the doors.