I'm not familiar enough with the factory head unit to know if it has RCA outs on it...doubtful. You could prolly go the OG way, and use the rear speaker outputs as a high level input, or buy a speaker level to RCA converter. The premium Alpin audio has a dedicated amp mounted on the rear deck in the trunk. You maybe able to decipher the wiring schematic on the input, those should be low level outputs, and you would be able to splice your own RCA's into those wires. Wish I could help more, but I haven't taken a close look at the audio hookups yet.
There has never been a factory radio with preamp outputs. You can do a few things. You can unplug your inputs at the factory amp and install RCA ends on them, but only if you are going to run an amp to front and rear speakers. You can also use a line output convertor, this would have to go on the output side of the factory amp. There are also some companies that make a better convertor that will allow you alot more flexability. Aamp of America, Memphis, Alpine and Audio Control all have this.