I think you would want to bypass the factory amp. as far as the HU and the RCA preout go...id buy a new HU, the factory ones suck . i dont know how you would hook it up w/out it having the preouts on the HU.

'99 ČoňtôúŔ SE Kenwood KDC-MPV7019 AudioBahn AW1206T Flame Q Sub AudioBahn 1300HCQ Amp Blue LED Switches and HVAC