lost, amp to stock head unit?

if your keeping the factory head unit the easiest way is to use a male and female version of the wiring harness for your car (Metra 70-1721 and 71-1721) and 2 line out converters aka stepdowns. take the 2 harnesses and connect everything up to each other but the 8 speaker wires. connect the LOCS to the radio side speaker wires and then connect some speaker wire to the car side harness and run them to your amp. that will give you RCA output from the radio and connection  from your amp to the speakers