Hey mate, there are at least 5 threads that I know of about this exact topic in the audio section and ve section. At least 2 on the first page of the audio section (not including yours). Have a bit of a search through as all your questions and more have been covered in detail in these other threads.
Here's one of em
As has been said in all the other threads...Your best option is to use an LOC. Which allows you to keep your factory head unit and all the functionality. It just converts the speaker outputs from the H/U into RCA outputs. You can then run your RCA leads from this to your amp.